When we learned how shadows can tell the time.
There is no better way to learn than by making and doing. Our teachers have that concept in mind every time they get ready to teach a lesson.
Recently our third graders made an experiment with a small branch and a bunch of stones, they had to stand the piece of wood using nothing but the rocks; after that, they had to make a mark with chalk on the floor, right where the shadow was.
What a surprise they had when returning after an hour and seeing the shadows had moved.
Now they have a nice story to tell to their grandchildren, from when they learned how to make a clock with nothing but a branch, some rocks, and the shadow.