Without a doubt my favourite interview logic question. Typically, you'll be asked this question in person around the second interview, and asked to explain your thinking as you work your way through it.
You'll normally be given somewhere between 10 and 15 minutes to work it out.
(This question is slightly altered from its original form to hinder recollection)
Suppose you're on a game show, and you're given the choice of three containers.
You pick a container, and the host, who knows what's in each container, opens a different one.
The container he opens has a $10 bill in it. He then says to you:
“Do you want to change your choice, or do you want to stick with the container you chose originally?”
Is it to your advantage to switch your choice?