Diving bell

You can immerse a pocket-handkerchief in water, without it getting
wet: stuff the handkerchief firmly into a tumbler and immerse it upside down in the water.

Air is certainly invisible, but it nevertheless consists of minute particles, which fill the available space.

So air is also enclosed in the upturned glass, and it stops the water entering. If, however, you push the glass deeper, you will see that some water does enter, due to the increasing water pressure, which compresses the air slightly. Diving bells and caissons, used under water, work on the same principle.