Answer:
basketball or other large bouncing ball
A tennis ball or other bouncing ball that is smaller and lighter than the bigger ball
A ping-pong ball, smaller and lighter than both other balls (optional)
A hard surface to bounce balls, preferably outdoors, surrounded by a large open area where a ball can fly without colliding with people or objects
A helper (optional)
Camera that films video (optional)
Yard stick that measures meters (optional)
Preparation
Gather your test balls in the open area where you will perform the activity.
Note: If your ball does not bounce well, it might be deflated or old. You can add air to some balls like basketballs, soccer balls, volleyballs, etcetera. A bicycle pump with a special piece to inflate balls can help you do the job. Other balls, such as tennis balls or baseballs, wear out with use. Why do you think a deflated ball does not bounce well? Why would a well-used tennis ball not bounce as well?
Procedure
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First, hold the larger ball in front of you at shoulder height and let it fall without giving it a push. How high does it bounce back—almost up to your shoulder, your hip, your knee?
Repeat this a couple more times. Does the ball bounce back to approximately the same height each time?
Repeat the test with the smaller ball. Do you think it will bounce higher or lower then the larger ball? How high does this ball bounce back?
Repeat a couple more times. Are your results consistent?
Do you think the ball can bounce back higher than the height at which you released it?
Hold the larger ball in front of you at shoulder height like you did initially. Place the smaller ball on top of the larger ball so they just touch, and hold it there. You might need a helper to hold the balls in place. In a moment, you will release the two balls at the same time. What do you think will happen? Will the larger fall faster or will both stay together as they fall? How will they bounce back? Will a ball bounce higher, the same or lower than before?
Try it out and observe.