Lay the kitchen sponge flat on your work surface. Use scissors to carefully cut a small slit in the top center of the sponge. The slit should be just big enough to hold the straw upright.

Add a small dab of craft glue inside the slit, then place the bottom of the straw into the opening. Press it gently into the sponge so it stands as straight as possible. Set the sponge aside while you make the sail.

Cut a triangle out of card stock for the sail. Make the triangle tall enough to fit on the straw, but not so large that it makes the sponge tip over.

Use a hole punch to punch one hole near the top of the triangle and one hole near the bottom of the triangle. Thread the straw through both holes so the card stock sail sits on the straw.

Cut a small diamond shape out of card stock for the flag. Add a little glue to the inside center of the diamond, then fold it in half around the top of the straw. Press the flag together so it sticks to itself and stays in place.

Let the sailboat dry for about 15 to 30 minutes before placing it in water. Once it is dry, float it in a shallow pool, sink, bathtub, or small outdoor water table.