Ok.. moxnix seems to have handled questions quite well but the fly in the jar thing I would have to disagree.. my reasoning:

since the fly is flying around, the mass wouldn't change at all and thus no change in weight.. but let's assume the fly crocked and fell to the bottom and thus added mass to the jar...

weight = mass * gravity, where gravity is a constant of 9.81

Thus assuming that mass increases, it is obvious that weight will increase with it... so in order for weight to increase, the fly needs to be somehow on the jar... it cant be suspended in air because it then does not add mass to the jar thus making it irrelevant to the jar's wieght..

BTW, I'm taking most of this straight from my Physics book so I don't think it can be disputed..