Brainteaser

Today's brainteaser is about using logic to solve what at first sight seems an impossible problem. Can you come up with an explanation?

You are on an island in the middle of a lake. The lake is in a remote part of the country and there has never been a bridge connecting the island to the mainland.
Every day a tractor and wagon gives hay rides around the island. Puzzled as to how the tractor had gotten onto the island, you ask around and find out that the tractor was not transported to the island by boat or by air. Nor was it built on the island.

Explain how the tractor got there!