I remember that my math teacher at school was specially fond of logic problems, and each week he would give us a couple to solve.
You have 10 crates of gold coins that were salvaged from an old shipwreck. All crates have the same appearance and the coins look exactly the same. However, one of the crates has been swapped by a cunning thief and all the coins in that crate are almost perfect replicas.
The only difference between the replicas and the true thing is 0.01 grams: the good ones weight is 10g, and the bad ones 9.99g.
In order to find out the thief, you have a one-plate digital scale that works as follows: you load the plate, you press a button, and it gives you the weight of what was in the plate in the instant you pressed the button. You also have a felt-tip marker.
The solution here is finding out which crate is the fake one, which seems pretty obvious.
However, the big price is for the person that can find out which one is the fake using the scale as few times as possible
The best answer is that you only need to use the scale once.
You use the marker to assign a number to each crate, you pick as many coins from each crate as its number, and write the number on them.
Then you gather all the coins together and weigh them. The number of centigrams missing will give you the number of the fake box.