Why in division , there is sometimes a remainder ?

Because all numbers don't perfectly go into other numbers.

Say, for example, that you want to buy a skateboard that costs $50. You only have 3 20 dollar bills in your wallet, so you give the cashier $60 and he gives you $10 in change, because its IMPOSSIBLE to pay $50 exactly if you only have 20 dollar bills. 

In the box there were 4