Zak has a cold.
Well, Zak has a cold, I have a cold, and Raoul probably has a cold too (but it's so similar to his normal hayfever symptoms that he can't tell much of a difference). This cold entails a very runny nose, sneezing, a cough, and irritated eyes (at least for me).
Today as I was talking Zak into having his nap, I decided to give him a rudimentary lesson on the immune system. I explained that he was feeling sick because of the yucky germs, and that he needed to go to sleep so his white blood cells could fight the yucky germs and make them "go away" (not getting too technical with a two year old). I said that we all have germs but sometimes they make us sick and feel yucky, and then the white blood cells have to fight them. Zak listened to all of this and repeated parts of what I was saying, and agreed to go to bed so that the white blood cells could make the yucky germs go away.
It took awhile for him to go to sleep, but once he did we took a fairly good nap. The first thing he said when he woke up? "White blood cells pretty cool!!"
Science for the win!