OK, here is a classic one. I have even seen it as a brain teaser in interview prep material.
Just like the picture above, imagine that there is a scale in front of you. You also have 12 weights that seem identical to each other. The problem is that one of the weights has a different mass from all the others. You want to find this weight, but you don’t know if it is lighter or heavier than the others. Can you use the scale only 3 times to find out which of the 12 weights is different from the others? How many weights will you put on each side of the scale?
Wharton Undergraduate Statistics Society 