Nice one; how about a canister of helium then? heavier when charged or discharged?
This one raged as an argument thread for days IIRC.
A similar questiojn that I discussed on a thread a few months ago. I’m not convinced by the answer I got.
You eat a plate of beans. On their journey through your guts, bacteria work on them and seperate them into methane and heavier material. Do you become lighter as result? Do you gain weight when you fart?
Cecil says yes, but I’m not convinced. Does the methane displace air in the body? The gut may expand because of the gas, but does the body as a whole become less dense?
Although accurate science is always useful, there’s always one poster who lowers the tone…
Assume you have 500 canaries flying in a sealed truck.
What happens if they all fart simultaneously?
What happens if they all poop simultaneously?
I’m not sure that canaries ever do this, but assuming you’ve got magical farting canaries (I’ll be putting that phrase forward next time someone wants username suggestions)… not a great deal; the canary is pushed upward by ever so slightly by the jet of gas; the truck is pushed downward as its effect (if not the actual gas itself) reaches the floor; the truck becomes momentarily lighter, then momentarily heavier, then it goes back to the normal weight of a truck plus 500 canaries.
This might be easier to organise. Part of the system briefly goes into free fall; the truck is momentarily lighter while it falls, then momentarily heaver as it transfers its momentum to the floor, then it returns to the normal weight of a truck plus 500 canaries.
Charged. More molecules implies more mass. No (or negligible) change in volume means no change in buoyancy. Thus, charged canister is heavier.
Canister completely evacuated of all gas is lightest of all.
You’re right, of course - adding mass to a system of (more or less) fixed volume can’t ever make it lighter, but you occasionally find folks who just won’t accept it.
Thanks for that. Not only do I have some new science knowledge, but also a happy image to conjure up whenever I want! 
And what about intestinal gas? When you fart do you lose weight? Or gain it? Does intestinal gas change a human body’s volume?