Because it's raining

I was reading a book with a little boy (aged 5). It was about a football match...his passion. 

We were looking at a picture of Mum who was very very angry on the sidelines of the football pitch. Dad was stood there on the football pitch. He was playing and the referee was showing him a yellow card.

My question.
'Why is mum so angry do you think?'
Little boy studies picture for ages and ages and then says very seriously
'Well because it is raining!'

Silly me and there was I thinking it was because the ref had showed dad a yellow card!