What fills a soap bubble?

A soap bubble floats past, shining with tiny swirls of color. You can see right through its middle, almost as though nothing is there. What fills a soap bubble?

The answer is air, the invisible stuff all around you that you breathe. A bubble may look empty, but air fills the space inside. Even when you cannot see air, it takes up room.

To see how air gets inside, let's follow a bubble from the start. When you dip a bubble wand into soapy water, you bring some of that water up with the wand.

Across the wand's hole stretches a soap film, a thin skin of soapy water. The soap helps the water spread into this skin and stretch without breaking straight away. That soap film will become the bubble's outside.

Next, you blow gently through the wand. Your breath sends air against the soap film, pushing it outward. The film stretches into a little pocket, with your breath flowing inside.

Can you guess what fills that growing pocket: soapy water or air? Air fills it! The soapy water makes only the thin outside skin, not a whole watery ball.

As you keep blowing gently, you push more air into the pocket. The pocket grows, then its opening closes as it leaves the wand. Now the soap film surrounds the air on every side.

The soap film pulls inward, like a stretchy skin trying to get smaller. The air inside pushes back and takes up room. Together, those pushes and pulls give a floating bubble its round shape.

For a little while, the thin skin holds the air inside. When that skin breaks, the bubble pops and the air mixes with the air around it. Nothing needs to pour out, because the middle was not full of water.

Soapy water, soap film, a gentle puff of air, a closed bubble, then a pop. A soap bubble contains air inside a thin soap film.

The next time a shiny bubble drifts past you, you will know its quiet little secret: air is tucked inside.