The mystery of the sponge

Oliver seems to be keen on questions at the moment. "Who's that?!" is a popular one. Also "who's at the door?", whether there's anyone there or not. (Well, I don't THINK there was anyone, but there's often more logic to what he says than I can immediately grasp.)

Recently, while enjoying his evening bath, he found another question: "where's the sponge?" He had hidden it under his legs, quite effectively in fact.
"Where is it?" I asked him.
"Over there!" he replied, pointing behind himself to the top end of the bath.
I duly searched beneath the bubbles. "I can't find it."
"Over there!" he directed to the other end of the bath. I duly searched again. At which point... voila! He let the sponge float up from its hiding place, emitting a loud and satisfied chuckle as he did.

Then we started again.
"Where is it?"
"Over there!"
"I can't find it."
"Over there... Ha ha ha ha!"

And again.
"Where is it?"
"Over there!"
"I can't find it."
"Over there... Ha ha ha ha!"

And again and again and again. It just never got less funny for the little man. He didn't even need the pretence of surprise. "One more time..." he said after I failed to notice the sponge's umpteenth disappearance. How could I say no.