10 boxes numbered from 1 to 10. Each is supposed to contain 10 gold chains. Each chain weighs 100g, meaning each box weighs 1kg. But now you are told that one of those boxes has been tampered, and although still containing 10 chains, but each chain only weighs 99g, meaning a total weight of 10 chains for that box of 990g (10g lacking). For the present purpose, ignore the weight of the boxes.
Now you are given a sensitive scale. BUT you can only weigh ONCE, i.e. only ONE reading allowed. It's up to you to do as you like with the help of THAT scale. How do you determine which box contains only 990g?