A quick puzzle for you. It'll be relatively easy if you've seen Die Hard 3 :)
You have an unlimited supply of water. you've a 5-litre jerrycan and a 3-litre jerrycan. You've to get 4 litres exactly. How?
You're allowed to dump water, whatever you want, so long as you get 4 litres.
Someone answer in the comments for everyone else!