Collect everything you need to build a simple sundial: a flat base (cardboard, paper plate, or piece of wood—about 12 inches across), a stick or pencil (6-8 inches long—this is your gnomon, the part that casts the shadow), tape or glue to secure the gnomon, a marker, a ruler, and a watch or phone to check the real time. You'll also need a sunny outdoor spot where the sundial can sit undisturbed for several hours—a patio, driveway, or flat area of the yard works great. Make sure the spot gets sun from morning to afternoon (at least 9am to 3pm if possible). Explain to your child that people used sundials for thousands of years before clocks were invented. The sun's shadow moves as the Earth rotates, and we can use that movement to tell time.

Now build your sundial. Take your flat base and attach the gnomon (stick or pencil) upright in the center—use tape or glue to secure it so it stands straight up. The gnomon is what casts the shadow that shows the time. Place the sundial in your chosen sunny spot on flat ground. Use a compass (phone app works) to orient the sundial so the gnomon roughly points north. If you don't have a compass, face the sundial so the gnomon's shadow points toward you at noon—that's roughly the right direction. Mark 'N' on the north edge of the base and 'S' on the south edge so you can replace it in the same position if it gets moved. Make sure the sundial is level—if it's tilted, the shadow won't move evenly. Weight it down with rocks or tape it in place if it's windy.

Wait for a time on the hour (like 10:00 AM, 11:00 AM, or noon—noon is the easiest because the shadow points north then). Check the actual time on your watch or phone. When it's exactly on the hour, look at where the shadow of the gnomon falls on the sundial base. Use a marker to draw a line along the shadow from the gnomon to the edge of the base. Write the time next to that line: '10 AM' or '12 noon'. That's your first time mark! Explain to your child that the shadow is showing where the sun is in the sky right now. As the Earth rotates and the sun appears to move across the sky, the shadow will move too. In one hour, the shadow will have shifted to a new position, and you'll mark that as the next hour. For now, you've calibrated the sundial by marking one known time.

Now you need to mark more hours to make the sundial functional. Every hour on the hour, go back to the sundial, check the exact time, and mark where the shadow falls. Draw a line and label it: '11 AM', '12 noon', '1 PM', '2 PM', and so on. You'll need to do this over several hours to get a good range—ideally from 9 AM to 4 PM or later if the sun is still shining. Each hour, the shadow will have moved about 15 degrees around the dial (360 degrees in 24 hours means 15 degrees per hour). If you can't sit outside all day, mark a few hours now and come back later or even on another sunny day to fill in more. By the end, you'll have a functioning sundial marked with multiple time lines. Your child will start to see the pattern: the shadow moves steadily and predictably as the day goes on.

Now that your sundial is marked, talk about the science. Explain that the sun appears to move across the sky from east to west because the Earth is rotating. The shadow cast by the gnomon moves in the opposite direction (west to east on the sundial base) as the sun moves. Sundials were used for thousands of years by ancient Egyptians, Greeks, Romans, and many other cultures before mechanical clocks were invented. Modern clocks are more accurate because sundials only work when it's sunny and can be affected by the time of year (the sun's path changes with seasons). Test your sundial: wait for a random time (like 2:30 PM) and see where the shadow falls. Does it land between the 2 PM and 3 PM lines? If so, your sundial works! Celebrate that your child just built a functioning timekeeping device using nothing but the sun.