You cannot make your way through the lab and enter every room, ending on the exit, without backtracking at all. But there is one room you can enter twice. Check the original question here.


The key to this riddle is realizing that you do not have to pull the emergency switch in the first room, the entrance, and so that room will not lock down. You can leave it, and then return to it and go out the other door. And indeed this is exactly what you need to do so you can hit all 16 rooms and end in the exit room.

Once you know to leave and reenter the first room, there are eight possible solutions to travel through all the rooms and end at the exit. Here they are:

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Michael Stillwell
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Michael Stillwell

This problem is related to Hamiltonian paths in graph theory, which are paths through a graph that touch each vertex of that graph exactly once. In the virus-contaminated lab, you cannot achieve a true Hamiltonian path and visit each room just once. But, because the first room is clear, you can backtrack through it to get back on the right track and pull the emergency switch in every room, saving the world from agonizing death by ancient virus.

Come back next week for another riddle!

*See all of our riddles here.

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Jay Bennett
Associate Editor

Jay Bennett is the associate editor of He has also written for Smithsonian, Popular Science and Outside Magazine.