Riddle: you are standing next to a lake. you have 2 bottles ONLY. One of the bottles is a 5litre bottle and the other one is a 3 litre bottle.
you have an unlimited supply of water and you need 4 litres.
the bottles are not labled and you cannot estimate how much water a bottle contains.
Answer: first you fill the 3 litre bottle to the top and you add the 3 litres of water to the 5 litre bottle you then fill the 3 litre bottle up again and add it to the 5 litre bottle.
the 5litre bottle is now full and the 3 litre bottle has 1 litre in it. so you empty the 5 litre bottle and add 1 litre to it from the remaining of the 3 litre bottle once the 5 litre bottle has 1 litre in it, you fill up the 3 litre bottle and add it to the five litre bottle.