The itsy bitsy spider, and other terrifying things

This afternoon, Simon sang "the itsy bitsy spider," and then said "that's all I know."

I jumped in "the itsy bitsy spider, when up the water - oh hey! Simon turn around, I want to show what I did when you were a baby, you used to love this!"

A little excited, he turned around, showing me his back. I started the song again, walking my fingers up his back like the spider, and running them down his back for the rain.

"ugh, ugh, ugh!" he jerked away from me and turned around, defensively keeping his front facing me. I tried to reach for his back to see if he would like the rainbow motion of "up comes the sun." He ran away from me and hid in his room.

After some time and gentle coaxing, I got him to tell me through his tears that he was scared of the spider and what it was going to do. The poor kid thought there was actually a spider on his back. He probably didn't like the sensation, maybe to the point of distress, and already freaked out, the spider seemed real. Or it could be that he just took it very literally. After accepting that there was never a spider on his person, he switched to concern for the spider. "But what happened to the spider," he asked, tearing up again. It took a few minutes, but in the end, he trusted that both himself and the spider would be okay.

He does this sometimes, thinking a joke we are making is real, and he gets so upset that he cries and hides, and its difficult to convince him it was just a joke. Pretend that a train is unhappy or troubled, and he flails on the floor, weeping. And other times, he'll giggle and joke along with us.

It's hard to know what will set him off, why some moments pretending to take his kitty is silly and a start to good natured rough-housing, and other times it will get you tears and a kick to the face. I can't tell why he was so convinced the spider was truly crawling up his back or in danger of being washed away. What makes him forget about jokes and pretend and take everything so seriously? We might be able to work it out, but in the meantime, I'll refrain from singing about spiders.