Is a bubble a liquid?

My 2 1/2 year old asked me what a bubble was made out of. He has a habit of asking hard questions.

Obviously it’s a gas in the middle, but what state of matter is the “shell”? Not a gas, not obviously a solid, although it holds its shape like one, but doesn’t flow like a liquid.

I’m thinking it’s a very thin layer of liquid, with surface tension holding it together on both sides, supported in a bubble shape by the gas inside.

What say you?

Yep, surface tension is the key. With a bubble blown from a bubble-blower (which I guess is what initiated it) you have a slightly-higher pressure on the inside, counteracted by the surface tension. (The surface tension on the outside also pulls inwards.) Hence the balance needed to make a bubble.
(Actually, this is a vague stab at the physics of it, so maybe sombody will prove me wrong…)

That’s exactly right. In fact, if you look at the surface of a soap bubble carefully, you can sometimes see the liquid that composes it "flow’’ within the layer.

Is the bubble the ‘shell’ of liquid, or is it the gas inside? What of bubbles rising in a bottle of, say, soda water? In that case there is no ‘shell’ to bubble is only the gas.

That’s different - it’s a bubble within a liquid, rather than one on the surface. One in the liquid can reach the surface and burst, or it can reach the surface and become a bubble as has been described already.