Here’s a classic puzzle: you are a rather evil King. You have 1000 bottles of wine. Exactly one is poisoned. You need to find out which one. You have 10 prisoners. You can use them to test the wine.
Now, to make the problem more difficult: poison takes effect only after 10 hours, and you need to find the poisoned bottle within a day. (If the poison worked instantly, you could easily find the poisoned bottle by making just one prisoners test them all one by one. If you had unlimited time, you could cut the number of bottles in half while using just one prisoner by making him test half of the bottles, and it would be enough with 10 to find the right one. But you do not have time for this.)