Articles, Blog

Can you solve the virus riddle? – Lisa Winer

December 22, 2019

Your research team has found
a prehistoric virus preserved in the permafrost and isolated it for study. After a late night working, you’re just closing up the lab
when a sudden earthquake hits and knocks out the power. As the emergency generators kick in,
an alarm confirms your worst fears: all the sample vials have broken. The virus is contained for now, but unless you can destroy it, the vents will soon open
and unleash a deadly airborne plague. Without hesitation, you grab
your HazMat suit and get ready to save the world. The lab is a four by four compound
of 16 rooms with an entrance on the northwest corner
and an exit at the southeast. Each room is connected to the adjacent
ones by an airlock, and the virus has been released
in every room except the entrance. To destroy it, you must enter each
contaminated room and pull its emergency
self-destruct switch. But there’s a catch. Because the security system
is on lockdown, once you enter the contaminated room, you can’t exit without
activating the switch, and once you’ve done so, you won’t be able to go
back in to that room. You start to draw out possible
routes on a pad of paper, but nothing seems to get you
to the exit without missing at least one room. So how can you destroy the virus
in every contaminated room and survive to tell the story? Pause here if you want
to figure it out for yourself. Answer in: 3 Answer in: 2 Answer in: 1 If your first instinct is to try to graph
your possible moves on a grid, you’ve got the right idea. This puzzle is related to
the Hamiltonian path problem named after the 19th century Irish
mathematician William Rowan Hamilton. The challenge
of the path problem is to find whether a given graph
has a Hamiltonian path. That’s a route that visits
every point within it exactly once. This type of problem, classified
as NP-complete, is notoriously difficult when the graph
is sufficiently large. Although any proposed solution
can be easily verified, we have no reliable formula or shortcut
for finding one, or determining that one exists. And we’re not even sure
if it’s possible for computers to reliably find
such solutions, either. This puzzle adds a twist
to the Hamiltonian path problem in that you have to start
and end at specific points. But before you waste a ton of graph paper, you should know that a true
Hamiltonian path isn’t possible with these end points. That’s because the rooms form a grid
with an even number of rooms on each side. In any grid with that configuration, a Hamiltonian path that starts and
ends in opposite corners is impossible. Here’s one way of understanding why. Consider a checkerboard grid with
an even number of squares on each side. Every path through it will alternate
black and white. These grids will all also have an even
total number of squares because an even number times
and even number is even. So a Hamiltonian path on an
even-sided grid that starts on black will have to end on white. And one that starts on white
will have to end on black. However, in any grid with even
numbered sides, opposite corners are the same color, so it’s impossible to start and end
a Hamiltonian path on opposite corners. It seems like you’re out of luck, unless you look at the rules carefully
and notice an important exception. It’s true that once you activate
the switch in a contaminated room, it’s destroyed and you can never go back. But there’s one room
that wasn’t contaminated – the entrance. This means that you can leave it once
without pulling the switch and return there when you’ve
destroyed either of these two rooms. The corner room may have
been contaminated from the airlock opening,
but that’s okay because you can destroy the entrance
after your second visit. That return trip gives you four options
for a successful route, and a similar set of options if you
destroyed this room first. Congratulations. You’ve prevented
an epidemic of apocalyptic proportions, but after such a stressful episode,
you need a break. Maybe you should take up that recent
job offer to become a traveling salesman.


  • Reply Tran Le October 30, 2019 at 8:39 am

    First time ever i got it righttgtttttt

  • Reply CAT CAT October 31, 2019 at 12:16 pm

    Virus riddle? Lemme just use an antivirus to solve things up

  • Reply DRK JUJU October 31, 2019 at 3:42 pm

    I know how you don’t

  • Reply Mike Chen October 31, 2019 at 11:45 pm

    Can't you just take one for the world?

  • Reply Reptard November 1, 2019 at 4:23 pm

    i go down 1 pull lever, go back up to smileface room, go out 3, down 1, in 2, down 1, in 1, down 1, out 2, up 1, out 1, down 1.

  • Reply don't look aty channel pic November 2, 2019 at 1:29 am

    This is the easiest puzzle i found in this channel

  • Reply The Catss November 2, 2019 at 7:25 am


  • Reply Le Dung November 2, 2019 at 8:25 am

    When you say the first room is contaminated after the opening the other room and I’m like , what if you escape? The air outside is gonna be contaminated if you open the door!

  • Reply Captain A. November 2, 2019 at 10:03 pm

    Love that traveling salesman joke at the end

  • Reply Ande Yashwanth November 3, 2019 at 4:38 pm

    Wouldn't the virus come out when he open the exit door?

  • Reply Ryan Z November 4, 2019 at 12:30 am

    I finally got one right!

  • Reply CHOUDHARY_SAHAB November 4, 2019 at 6:31 am

    OMG I have solved it.First I thought it is impossible then I focussed on why I am not utilizing the entrance room and suddenly idea clicked .

  • Reply Some Guy November 5, 2019 at 8:56 am

    Bruh this one was easy

  • Reply The Optic November 5, 2019 at 1:52 pm

    The first Ted riddle I solved!

  • Reply Yuto Adams November 7, 2019 at 10:45 pm

    Why does the vents open anyway in the first place for a virus…

  • Reply Andrew Zhang November 8, 2019 at 8:05 pm

    Wouldn’t he release the virus into the exit after he opens it?

  • Reply Pie Plays November 9, 2019 at 1:17 am

    "Breached containment"
    SCP Enthusiasts: (heavy breathing)

  • Reply The Creeper way November 9, 2019 at 3:23 am

    The answer is vacsinat your kids

  • Reply dacreator 122 November 9, 2019 at 9:39 am

    69k likes. Nice

  • Reply Random Jay November 10, 2019 at 1:21 am

    Is that a shape?

  • Reply Don't call me Nymphadora November 10, 2019 at 1:52 am

    Just give everyone one of those suits

  • Reply Fakemankuku3 November 10, 2019 at 3:40 am

    I knew it first thing

  • Reply Omid Eftekhari November 10, 2019 at 5:25 pm

    TedEd-Can you solv-
    Me: NO I CAN NOT

  • Reply Miunu Gaming November 10, 2019 at 9:31 pm

    My answer was go to every row then after that go fown and do it agian without missing

  • Reply ankur pal November 11, 2019 at 10:22 am

    This one was easy

  • Reply Jeanie Basham November 12, 2019 at 8:42 pm

    I solved it correct

  • Reply Dizaster November 14, 2019 at 2:13 am

    I actually solved it by my self and before it told the answer

  • Reply Sir. Daequean Potato 46 years ago November 15, 2019 at 1:27 am

    Girl I woulda just moved to the Bahamas :3

  • Reply random person November 15, 2019 at 7:10 pm


  • Reply Entity Valkyrie 2 - Fixing Your Flags version 2 November 16, 2019 at 1:36 am

    Maybe build a lab 5×5 instead of 4×4 so that a hamiltonian path is possible

  • Reply Welfor Ramon November 17, 2019 at 2:08 am

    Person: Are you Co-Ed, Special Ed, or Advanced Ed?

    Me: I’m TED-Ed

  • Reply Jude Robinson November 17, 2019 at 5:44 pm

    What if I killed myself

  • Reply Dhruv V November 18, 2019 at 1:28 am

    I'm liking this video

  • Reply RainBowPanda November 18, 2019 at 11:27 pm

    Let’s just talk about the person who designed this room!

    They need to do a better job… at their job…

  • Reply Nguyen Max November 19, 2019 at 2:33 am

    I mean, they never said you have to close the door. Since they fill the room with cement then it could flow into other rooms. Then you can use the flow to you help and cover all the rooms with cement. Afther you can leave through the start.

  • Reply Apollo_ 909 November 19, 2019 at 4:28 pm

    No, I can’t.

  • Reply LexiShows Gacha November 20, 2019 at 12:17 am

    I figured it out without going back

  • Reply Jk It’s angel November 20, 2019 at 10:49 pm

    Go up and down

  • Reply Alice Dillon November 24, 2019 at 8:20 am

    Bruh you're in a hazmat suit. Just call the CDC and wait for them to arrive, then ur good.

  • Reply Bilnaz Ceylan November 24, 2019 at 12:16 pm

    başka bir yolla da son odaya gidilebilirdi yani demek istediğim ilk odaya iki kez itmeden de son odaya varabilirdik

  • Reply Ezra Ur Boi November 24, 2019 at 4:23 pm

    or you can go to your local gun store and kill the virus – it is living right?

  • Reply Анастасия Пузанова November 24, 2019 at 9:15 pm

    My decision was to go through all contaminated rooms, finish at the start and leave through the entrance. It seemed SO obvious. PLEASE tell me if it counts, I'm literally dying to know. It would be the second TED ridle ever I have ever solved

  • Reply ThunderShock November 25, 2019 at 8:04 pm

    Die for the planet.
    Be the saviour

  • Reply Twin Seption November 25, 2019 at 10:08 pm

    This is the first riddle i got right

  • Reply SrpntHous2 November 26, 2019 at 5:59 pm

    Deadly virus about to be released

  • Reply Ronald Matos November 29, 2019 at 1:50 am

    Me: (solves riddle)

    Also Me: (gets trapped because the exit door was jammed)

    Also Also Me: FML.

  • Reply RazR Chillz November 29, 2019 at 1:06 pm

    This is the first one of these Ted riddles I’ve actually solved

  • Reply Boix Boiz November 30, 2019 at 12:24 am

    I did it!

    Because i’m not a mouse.

  • Reply David Jr December 2, 2019 at 2:19 pm

    Yay I got it right !

  • Reply nikunj gupta December 2, 2019 at 3:37 pm

    So brilliant to put that easter egg sorta thing about Travelling Salesman at the end. Brilliant!

  • Reply DragonSoul龍 December 4, 2019 at 9:45 am

    this is the only riddle of this channel that i solves and im not proud at all

  • Reply Feben H December 4, 2019 at 9:39 pm

    Well that took me about 5 seconds. Good riddle. Sigh.

  • Reply Elizabeth Urbanczyk December 8, 2019 at 1:38 am


  • Reply Blue Vaquita December 8, 2019 at 7:54 pm

    bruh seriously just shut down the power and the air vents won't open

  • Reply LaZYer December 9, 2019 at 10:29 pm

    bruh, just disable the ventilation sistem

  • Reply Dacco December 10, 2019 at 8:14 pm

    Wait. A pre-historic virus? Weren't
    a) diseases weaker because they haven't developed against humans OR vaccines and medicine?
    b) animals didn't even look like humans so diseases are going to have a hard time understanding human bodies?

  • Reply majesticshrimp December 10, 2019 at 11:53 pm

    I got it riiight yeyeyyy

  • Reply Brandi Williams December 12, 2019 at 2:17 pm

    cuz the vents open

  • Reply Limitless Beast December 13, 2019 at 8:45 pm

    This is basically the snake game.

  • Reply TOLAEN December 14, 2019 at 4:57 pm

    I dont get viruses

    The viruses get me

  • Reply Dark Obito 14k December 18, 2019 at 11:12 am

    Wait so he couldn’t just go around to every room starting with the outside going in and than going back into the start and exit from the start?

  • Reply VenomROBLOX December 18, 2019 at 2:55 pm

    Well i have different way to make but still doesn't need the entrance twice 🙂

  • Reply Doupis December 20, 2019 at 5:59 am

    I found it after watching tons of your riddles

  • Reply Simit Devi December 22, 2019 at 12:10 pm

    Go in a pair keep one friend at entrance and go horizontally then vertically down then up then down then up thats how u reach rntrance again now go outside as u hv kept ur friend outside enjoy!!!

  • Reply Nasreen Habib December 22, 2019 at 4:07 pm

    BTW we can solve this riddle without going back into the entrance… →↓→↑→↓↓←←←↓→→→↓←←←←↑↑↑….. by this way u can also destroy the rooms nd go back to tge entrance from where u can from…
    Match my arrows with the graph up there

  • Reply Abeast Gaming December 22, 2019 at 7:24 pm

    I would have an emergency self destruct switch to destroy all the rooms at once

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