What do I need to build a sand castle?

Hint 1 of 5:

Actually, *you* (Ernie) can't build a castle.

Hint 2 of 5:

But one of the Phart brothers can.

Hint 3 of 5:

He will need a tool or two to help him.

Hint 4 of 5:

Look in the hardware store.

Hint 5 of 5:

There is only one item that will help.