Quote Originally Posted by Wyvern View Post
Old one but maybe no one knows it.

You have two empty containers – one has a capacity of five litres of water and the other can hold at most three liters of water. Both containers are made of clear plastic and have absolutely no markings anywhere. Here’s your challenge: given an unlimited supply of water, I want you to come up with a way to measure exactly four litres of water.
Fill up the 5 litre bottle then pour the water from that into the 3 litre bottle, you'll have 2 litres left in the big bottle. Empty out the small bottle, then pour the 2l into the small bottle. Now you have 2 litres in the 3 l bottle. Now fill up the 5 litre bottle again then use that water to fill up the 3 litre bottle, you will now have 4 litres left in the big bottle.