It is VERY DANGEROUS to look directly at the sun. NEVER LOOK DIRECTLY AT THE SUN!

The sun can be observed by using a pin or a small nail to
punch a hole in the center of a sheet of cardboard. With
your back to the sun, focus the sun's rays through the
hole onto another sheet of cardboard. The tiny image cast
will be that of the sun.

Any small telescope with an external focal plane can be used in a similar way to project an image of the sun. During a solar eclipse it is possible to watch the image gradually become obscured by the moon.

Here's another way to watch it: Put a dishpan or other large container of water down onto the ground. Stand beside it, making sure your body does not cast its shadow into the pan. You can look into the container and see the sun reflected on the water. As you watch, you can see the reflection of the sun become obscured by the moon.

Caution: No direct observation techniques for viewing the
sun are safe. Looking through exposed photographic film, sun glasses, smoked glass, or similar materials is also unsafe. The only advised technique is to view the sun indirectly using a pinhole device, or the pan of water.

I think the intense light of the sun (and probably other
lights, too, such as lasers) burn the retina of your eyes.

The brilliant scientist says: "Direct viewing of the sun,
even for a short time, can cause burns to the eye's retina.
Burns will not be felt but can produce a permanent blank spot in the field of vision."

Using a frosted bulb as a light source, let the light from
the light source pass through a pinhole in a card to form a
small image of the light a yard or more away.

Draw a pair of similar triangles on the chalkboard to diagram how the image is made. Calculate the size of the bulb, then measure to compare the actual size to the calculation.

Now let sunlight pass through a pinhole in a card to form a
small image of the sun a yard or more away. Draw a pair of
similar triangles on the chalkboard to diagram how the image is made.

You can see from the diagram that the ratio of the image's
diameter to its distance from the pinhole is the same as the ratio of the sun's diameter to its distance from the earth.