A puzzle (2)

A surface, bounded by the trefoil. You can make such a surface easily yourself. Take a strip, twist it three times, and glue the ends together. Voila.

But there's something funny with this surface. Try to paint the two sides in different colors. Then you find out there are no two sides. Next puzzle: Can you think of a 'normal' surface, with two sides, that is bounded by the trefoil? For a solution, click here.