Daily Brain Games (Dec 25) - how clever are you?

Here is the information :

You have 10 bags with 10 in every bag. Each ball has a weight of 10 gramm. Only in 1 bag the balls weights are 9 gramm each.

Here is the question : You can do whatever you want ,for example put the balls from one bag in the other. You only have the chance to weigh them 1 time . After this you have to tell in wich bag the balls with 9 gramms are.

I will post the answer later ^^