# Initiation Puzzle

A sealed bottle containing flies is placed on a scale. When does the scale register the heaviest weight : when the flies are resting on the bottom of the bottle, or when all the flies are in flight?

## 2 Answers

The scale registers...

...the same.

The insects in flight are held up by the air, and the air exerts weight onto the bottom of the bottle. The whole system is self-contained.

But when a fly takes off or lands, there will be a momentary change in weight, because of the force exerted by the fly against the glass: action – reaction.

• More generally, there will be rot13(zvabe jrvtug syhpghngvbaf jurarire gur syl nppryrengrf naq gur thfgf gung cebqhprf uvg gur gbc be obggbz bs gur obggyr). Jul 26, 2023 at 19:04
• what if all the flies are flying up against the glass? That would make it slightly lighter, no? Jul 30, 2023 at 14:29

My guess is that in both cases (at rest, in flight) the measured weight would be the same because the weight of the flies does not change as their positions change.

• Isn't weight a function of distance from the earth? Jul 26, 2023 at 12:26
• @msh210 Yes, but that would be rather nitpicky since the change in weight between ground level and around 5 feet up is negligible. However though, the OP might want to trick you with that, to which I respond xkcd.com/169. Jul 26, 2023 at 13:52