Why do they take things so literally?

Why do they take things so literally? I would say to get away with whatever they can. But I sometimes can’t help but laugh. Today, at a birthday party, Bo picked up someones pop can and went to take a drink. Quickly, I said, “Bo! No, don’t put your mouth on that.” Sensing my urgency he immediately pulled his head back. And I repeated, “Don’t put your mouth on that, o.k.” So he leaned down and stuck his tongue out to touch the opening on the top of the pop can, slobbering all over it. His mouth was not touching it at all, of course now it was covered in drool. Thank goodness the drink was already finished. As frustrating as it can be when he pushes the limits, sometimes I just can’t help but laugh.