[Ramius]

I still think there’s no way that’s enough helium to lift even a 30 lb boy.

If helium lifts about 1g per cubic meter of volume, a 30 lb kid is 30*16*28 = 13,440 grams, or roughly 13,000 cubic meters of He. Unless my arithmetic is way off... that 20-foot balloon couldn’t be more than a couple dozen cubic meters.

He may have climbed in, and the thing shot up in the air when he got out. He’s probably hiding somewhere afraid that he’s in trouble for letting dad’s cool balloon get loose.

[crusty old prospector]

Now that is what we need - some hard science.

[green iguana]

Helium has to lift way more than one gram per cubic meter.

[Daus]

I still think there’s no way that’s enough helium to lift even a 30 lb boy.

I agree. I don't buy this at all.

[Sloth]

If helium lifts about 1g per cubic meter of volume

I agree the boy wasn't in there, but there is no way your number above is correct. 1 gram is about the mass of a paperclip. A regular kid's helium balloon, with a volume much smaller than a cubic meter, can lift more than that.

[MikeGranby]

He2 lifts 27g/mol, and at STP one mol is 22.4 lt. To lift 30lbs or ~13,600g you’d need about 500 mol, which is about 11,300 lt or 400 cu ft, if I math is right. That’s a cube about 7’4” on each side, which isn’t too big. Now, you’ve got to allow for the structure itself, but it looks do-able to me.