Teacher had two bottles that were exactly the same. Both had a balloon in it and the balloons were exactly the same. The strange thing was that one balloon was impossible to blow up while the other balloon was quite easy to blow. We had lots of guesses as to why this might be so.
Each bottle looked empty but was full of air before the balloon was added.
What we didn't know (but soon guessed) is that one bottle had some holes in it so when we blew up the balloon the air in the bottle could escape out the holes and the balloon was easy to blow up.
The other bottle had no holes in it, so even though it looked empty it was full of air, and this air had nowhere to go when we tried to force more air into the bottle so this balloon was impossible to blow up.
We now know that even things that look empty are usually full of air!