A little riddle

I read a riddle the other day, and I wanted to make sure a few people got it wrong before I posted it. Now that I'm sufficiently pleased with the responses, I'm confident you'll get it wrong, too. Here it is:

A man is driving his son to school when they're in a car crash. The man dies, the son is rushed to intensive care. As he's wheeled into theatre, the surgeon shouts 'that's my son!' 

How does that work?

No peeking....

The surgeon is the boy's mum! I was baffled by this when I first tried to figure it out, so if you get it right I'll be impressed. I just goes to show that, as much as you regard yourself to be open minded, we do still stereotype!