Painting an aeroplane

This is my 5 year old son Shishir (SLA). Below is our conversation before going to bed:

Me: Good Night
SLA: No tell me a joke
Me: ok. I'll tell you the aeroplane painting joke. It goes like this: A stupid man asked another "aeroplanes are too big - how do they paint them?". The other replied "When it goes up, it will become small. Then they paint it"
SLA: Ha Ha Ha ... They are really stupid. How can they write many things on the aeroplane when it is too small!
Me: So the aeroplane really becomes small when it goes up?
SLA: The outside becomes small. The inside is same size because it is not moving.