The tree that wasn't there

This Sunday there were two kids digging a pit on the beach.

"What is this hole for?", I asked them out of curiosity.

The younger one showing a plastic bag filled with shells and sand replied "we fill the hole with this and then plant a tree in it. Tomorrow the sapling will grow into a full tree. We can come and sleep under it."

I looked around and found no sapling. "Where is the tree?", I asked.

"We will find one and bring it"

"Why the shells?"

"Silly, doesn't the tree need to eat something to grow?"

Digging holes. Reserving food. If only they could find the sapling!