Although you probably know that liquids and solids have weight, you may not have realized that air has weight too.
You can see this weight by taking an “empty” soda bottle, pumping out the air with a small pump, and covering it immediately. Then weigh the bottle.
Now, unscrew the cover and let the outside air fill the bottle. Weigh it again. You will find that the bottle now weighs more. So, the bottle that looked “empty” was really full, full of air.