While driving in the car with Jacob a few weeks ago, I suddenly heard "AHHHHHHHHHHH!" (very loud, fake scream). I thought something had bitten him at first! When I asked him what was wrong, he said, "I see a monster!" So I said, "Tell the monster 'Go away!" and Jacob says, "Go away, monster!" Then I asked, "Where did the monster go?" Jacob replied: "He went to work!"
This has become a new obsession, the monsters. They seem mostly benign and intend to do no harm, just to scare, but Jacob's fake cries of alarm can be, well, alarming. Especially while driving. Still, he will calmly tell them, "Go away!" if I remind him of this sacred duty.
I have found a use for these mysterious imaginary creatures of Jacob's own concoction: their absence. "Monsters don't like a clean house. They'll go away if we keep it clean." Jacob replies, "Let's go vacuum!"