My physiology class has been very enlightening for my 7 year old son. My book was opened to a page showing the progress of a baby moving from the uterus, through the birth canal, and out into the world. Fortunately, the picture didn't involve the mother (on this page), simply the baby.
My little guy looked at the picture (this isn't the exact one, but close), and said "Wow. So that's how I got squished out. No wonder I get headaches!!" There you go - migraines explained. Andrea (age 5) chimes in from the table across the room "Yea, I got squished out too!" Then Derek turns to me and says "Now all I need to know is how I got IN there." I told him that I would be happy to explain how he "got in there" anytime he wants to know. He thought it over for a second, then said "I think I need to be a lot older before I find out about that." He turned and wandered away. I have been giggling about it ever since!