Geometrical Optics

Try it: Focal point of a water drop.

You can easily determine the focal point of a water droplet. Make droplet at the end of an eyedropper or a drinking straw. Bring a piece of paper behind the droplet to catch the sun's rays that have passed through the droplet. Bring the paper closer until you have the sharpest image.
(The paper will then almost touch the water, so view the sun's bright image from the side so you don't block the sun's rays with your head.)

How far is the focal point form the water droplet?