To Make Ice Balls
You may notice that it is difficult to fill the ice balls to the very top or you may notice that water leaks out from the top holes.
This is by design. When water freezes, it expands. To get a perfectly formed ice ball, fill the tray to the designated rim and place the cover on top. Some water may escape through the top holes. This is OK. When the water freezes, it will expand, filling the entire ball.