Fill containers with water and put them in the freezer. Shallow containers (left) were easier to freeze and easier to blow toward the finish line. After a few hours, when the ice is slushy or partly frozen on the top, stick the straw flags into the container. Let them freeze the rest of the way.
After the water is completely frozen, put the bottom of the container under running water to loosen the ice boats. Put them in a baby pool, bath tub, or sand table filled with water for a race.