What do babies dream about?

I've often wondered this, given that my bubba is 3 months old and hasn't exactly experienced a whole lot of things, what exactly does he dream about? Sometimes when he's sleeping he smiles, which is totally cute, and sometimes he suckles, so maybe he is hungry? But last night I think he was having bad dreams :( he was crying and whinging and grizzling even though he was fast asleep. I ended up getting him out of his bed and hugging him for ages. But what could he possibly be dreaming? Makes you wonder!!