Huw blows air into the long balloon. The air spreads out to fill the whole balloon, right to the tip.

Huw blows air into the round balloon. The air spreads out to fill the whole balloon.

The air spreads out to fill the whole balloon, long or round.

The air has no shape of its own. It takes the shape of the whole balloon.

Now consider a bike tire.

A bike tire pumped up firm.

Huw pumps air in. The air spreads out to fill the whole tire, all the way round.

Now consider a room.

The air spreads out to fill the whole room, from the floor to the ceiling.

You can breathe in every corner, so air is in every corner.

The air spreads out to fill the whole room.