Seeing is believing

A few weeks ago, a robin built a nest on top of the light outside our garage. When K & D were here, we set up the ladder so they could climb up and see the eggs, and a few days later, the baby birds.
After the babies grew big enough to fly, the birds abandoned the nest, and I took it down and showed it to the kids. We noticed that the mother bird had used grass, mud, sticks, and even a little piece of plastic, and we all marveled at how the bird could do that, using just her beak!
A few weeks went by, and then today we were at the playground. After swinging on the tire for a while, K got off and announced that he was going to build a nest. He sat down on the ground and made a little hollow in the wood chips, and then carefully filled it with grass and a few small sticks. But he wasn't satisfied--he said it wasn't a real nest, because it didn't have any pieces of plastic in it!
I tried to explain to him that a nest didn't have to have plastic in it; that the bird just used whatever she could find. K was doubtful, but finally picked up a few scraps of paper lying nearby. But I could tell he wasn't convinced--the nest he saw had plastic, so that means that a REAL nest has to have plastic!