So my 5yo and I are watching youtube about octopus and mantis shrimps. (It is water related... ish) 
We read about how Mantis shrimp can see more wavelengths of light than we can.
What's a wavelength Dad?
Find images of wavelengths to show how small the spectrum we can see is.
Explain how white light can be split into colours like a rainbow.
The colours we see are reflected or absorbed.
Explain that light is both a wave and a particle.
So Dad... your white coffee cup is reflecting all the colours?
Yes.
And black things absorb all the colours?
Yes, not all but most.
So if light is a particle and black things absorb it, why doesn't it fill up and become bigger? 
Ummm, Is it time for lunch son? 
I'm sure when I was 5 I was still figuring out what flavour Clag I liked to eat while gluing my hands together... 
Mantis shrimps are very interesting creatures. Some have spears, some clubs, either way them striking's the fastest accelerating thing in the animal world - approximately the same acceleration as a .22 bullet.
Anyway, love a good question like that.
I would hazard a guess that the answer is that technically it does get bigger, as light is absorbed as heat into the coffee (or your skin for example), and heat expands things, so it would get bigger but just by a tiny little amount.
Select to expand quoteMark _australia said..
Schroedingers equation.
5yo in a box? I think there are Government agencies that frown on such experiments.
Select to expand quoterod_bunny said..
I'm sure when I was 5 I was still figuring out what flavour Clag I liked to eat while gluing my hands together...
I'm 43 and I'm still yet to fully decide on best flavour 
May I suggest that you teach your son the 'correct' theory of light and dark.
lightandsound.net.au/darksuckerpage.html
Q.... What is the difference between LIGHT and HARD?
A...... You can fall asleep with the light on!!!!!
Select to expand quoteCambodge said..Mark _australia said..
Schroedingers equation.
5yo in a box? I think there are Government agencies that frown on such experiments.
No see, everybody missed his most fascinating work - his equation. I was actually being serious ish
Schroedinger's equation describes how something can have both a wave nature and a particle nature. Most applicable to electrons I thought, but I did fail that part of uni
(well failed most of it, but anyway...)
So to my mind, if something has both a particle nature and a wave nature, it does not necessarily have the full set of properties of each.... ie photons do not "fill up space"
after our eldest daughter had lived with her younger sister for about two years. so she would have been about four at the time .
she asks " can i have a boy sister next time ?"
yes we did have a giggle about that question !!!! 
