#2 Drink Lid
If you write for enough years, you'll eventually have a second story about drink lids.
I picked up Eli 11.6 from school today. "Hey, I have a puzzler for you," I said, as we were walking to the car.
"What is it?" he asked.
"Can't tell you until we get to the car," I said. "There's physical material involved."
We reached the car and he got in. "Okay, what is it?" he asked. I handed him the drink lid.
"A lid?" he asked. "What am I supposed to do with this?"
"What's the single thing about P. Terry's that's annoying?" I asked.
"Wobbly tables," he said.
"And how do we fix them?" I asked.
"The paper that comes on the trays," he said. "Or sugar packets."
"But we usually have to do it more than once, right? Because when we use paper, if it isn't exactly the right thickness, the table will still wobble."
"That's right," he said.
"So I was sitting at a wobbly table today, and I went to get a drink," I said. "And when I picked up the drink lid, something clicked and I realized we've been using wrong material. Tell me why a drink lid would be better than using paper."
He looked at the drink lid carefully, turning it to different angles. "Well, it's not a smooth surface," he said. "Could the part of the lid that hangs over help somehow?"
"A good idea, but not quite," I said. "Look at the pattern on the lid."
"It's an 'X'," he said.
"That's from being folded in half, then being folded in half again," I said. "Now tell me this: how is folding plastic different from folding paper?"
"It's harder to fold," he said.
"It is, unless the paper is really thick," I said. "Plastic of this thickness has a resistance to being folded, so even after it's folded, it wants to return to its original position. So if you fold it, then fold it again, you almost wind up with--"
"A spring!" he said. "It's like a spring."
"That's right," I said. "So even if the folded plastic is a little thinner than the gap, the memory in the plastic will push up the shape until it fills the gap with the table. It even provides positive pressure upwards."
"That's neat," he said.
Wobbly tables have bugged me forever, and I should have thought of this forty years ago.