To warn boaters of shallow or rocky spots, the channel markers used by the Coast Guard must stay upright in the roughest waves. You can show your child how a little extra weight can keep a marker from tipping over by helping him fashion a small buoy.

MATERIALS

•Ping-Pong ball
•Modeling clay
•Drinking straw
•Small craft paper triangle
•Craft glue

WHAT TO DO:

Using a craft knife (a parent's job), slice the top off the ball. Next, cut a hole (big enough for the straw) in the center of the top. Ask your child to fill the bottom of the ball with clay. Glue the top of the ball back in place. Make a small slit in the top of the straw and insert the long edge of the paper "flag." Push the straw's other end through the hole in the ball.

WHAT'S GOING ON AND WHY

Your young experimenter can now test the buoy in a sink full of water. Encourage him to try tipping over the buoy by making some waves. Explain that the weight of the clay (called ballast) pulls downward into the water, keeping the top of the buoy upright. Compartments in the bottoms of large boats are also filled with ballast, such as lead or even water, to help keep them stable at sea.

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