For this craft you will need inexpensive sponges in two different colors, a tray for blotting paints and two colors of paint.
The objective here is simply to give kids a picture of how bricks were oriented around an arch while teaching the vocabulary "keystone" in the context of building.
Cut out your sponges as shown.
Use the sponges like stamps. Guide your child in putting them in place.
Talk about where the ground is and ask questions like,
"Can the stones float?" and "How do you think they got the bricks
or stones to stay in place?"
Describe how forms were used if they don't come up with the idea themselves.
If they do say something like, "they must have built something to keep it there."
Say, "yes! and it is called a form"
Explain that once the keystone was in place the form could be removed because it helped to
spread all the pull of gravity across all the parts of the arch which kept everything together.
THEN do the "Build an Arch" lesson so your kids can experience the real thing for themselves.