Place the lid on your container. If using a soda cup, put a drinking straw through the lid's center hole. Push the straw in until it touches the bottom of the cup.
If the lid on your container does not have a hole, punch one at the center of the lid. The hole must just be large enough for a drinking straw. Place a drinking straw through the hole.
Cut out a paper triangle to make a sail. You can use plain paper and decorate your sail with crayons or colored markers. You can also use paper with colorful or interesting patterns on it like giftwrap paper, origami paper or magazine pages.
For a more water-resistant sail, you can cut out your triangle from plastic bags or wax paper.
After a few hours, check on your container. Once the water inside is fully frozen, take the container out of the freezer.
Carefully remove the masking tape around the drinking straw and open the lid. Wait a few minutes for the ice to loosen or you can run the tap around the outside of the container to loosen the ice.
A big sailboat will need a larger block of ice. Use a large plastic container to freeze the ice. If the container does not have a lid, add a strip of cardboard to hold the drinking straw in place.
Punch a hole at the center of a 2-inch wide strip of cardboard. Position the strip at the center of the container. Bend the cardboard where it touches the top edge of the container. Tape the ends of the cardboard in place. Place a drinking straw through the hole.