How to explainn volume to a 10 year od

Show them a glass of water. 
tell them that the water is obviously in 3d because its not flat. Because this water takes up space in this glass it has a volume 

Say anything that's 3d has a volume, even a 3d movie possibly. I know SPY KIDS 3 came out and some of the characters or things would jump off the screen. Since they aren't flat like a normal tv they have a volume

And mathematical volume is (length x width x height) and I swear I didn't copy this and definition of volume: is something you can put something in or volume as in how loud or quiet an object is hope this helps you. :)