Sundial
Sundial is a tool that tells time by tracking where a shadow falls as the sun moves across the sky.
Think of it this way
A round cake pan sits in sunlight on a table. A candle in the center casts a shadow onto the pan's flat base. As the sun moves, the shadow rotates and points to a new spot on the pan. Each spot marks a different time of day.
Explaining sundial by grade level
When you set a stick in a paper cup in the sun, it makes a shadow. As the sun moves, the shadow moves too. You can mark where the shadow points each hour. Those marks help you tell time without a clock.