"I wouldn't have to walk if I could float."

"True, but I don't recommend trying it, there are lots of things in you that do more than just let you walk around." Lynn pokes her nose affectionately.

"Okay."

"So!" says Lynn, and she retrieves a pin to pop the balloon. "When the helium isn't there anymore to make the balloon lighter..." Pop! Whzzz, hsss, flop, goes the now deflated balloon. "It sinks to the ground."

"Why does it leave so fast?"

"Well, because the balloon was keeping all of the helium bottled up under pressure inside it. So the minute the helium saw an opening, it rushed out because it - didn't want to be under pressure and trapped anymore."

"It sees things and wants things?"

"Not the way we consciously do, but - matter wants to reach an equilibrium. While the helium was trapped in the balloon, there was an equilibrium inside it, and in the air outside of it. But when a hole was made in the balloon, there wasn't an equilibrium between the two, and they could interact with each other, so it reacted accordingly to fix it."

"But they could both interact with the balloon."

"Well, certain objects are - more resistant to going back to equilibrium than others. Because the - pieces that make up the object are sort of... Bonded. Or connected. So they don't easily separate, and they don't easily let things on each side pass through the surface."

"What does that have to do with the interacting with the balloon?"

"Well, they do interact with the balloon, but they can't get the balloon to let them through to interact what's on the other side of the balloon very much. But, the balloon being inflated was the helium inside interacting with it, keeping pressure inside it to make it bigger than it normally would be."

"And the air outside it didn't push back?"

"It does, actually." Lynn smiles. "And you're quite clever for asking that question. But the pressure outside was lesser than the pressure inside, so the helium was - gaining more ground."

"But there's lots more air than helium. Why can't it push harder?"

"Well, because the air is everywhere, and it's not concentrated like the helium was. If the air were concentrated into a small space, like the balloon, it would certainly win, but with it all over everywhere, that's not so. It gives small amounts of pressure everywhere, instead of concentrating on the balloon."

"Is it pushing on me?"

"Yes," says Lynn. "And me, and the chairs, and the walls, and the trees outside. But we're so used to it that it feels like nothing at all."

"What would happen if it stopped?"

"Well, it would be bad for you, because the things that make up you are used to pushing back against the air and wouldn't know how to stop. It would also mean that you wouldn't be able to breathe, which would be bad for - obvious reasons. That's why astronauts wear space suits, because there is no air pressure in space."

Astrid hmmmms contemplatively, then takes a deep breath and holds it.

Lynn waits, patiently.

"When is it useful?" asks Astrid.