If you have 10 bags each containg 10 pieces of eight. ...?

all eights in all bags weigh 1 kilo each, except for the eights in one bag - they are fake and weigh only 900 gr. each. You have the right to measure weight only once, but you can open all bags, mix eights, do whatever you like with them. How woul dyou tell in which bag there are the fake ones? Remember you can only measure weight ONLY ONCE!