It would be "Chess"
On six feet, I am a noxious drink, Of whose effects you shudder to think. Change only my second foot, and then You convert me into a horrible den Where the culprit, who gave the noxious drink, Awaits the fate of which you shudder to think.
There is a tunnel. Inside is only one train track. A train comes in from the left at 2:00 and another train comes in from the right on the same track at 2:00 also. Both come out safely.
How was this possible?