For which grade? And you want to make model of what?
You can create model of an object using strips of newspaper. Use some clothe hanger wire to make a frame. Cover it with paper mache made with glue. When it dries then you can put colors on it.
The details depend on what that model should show?
If you only have to show heart expanding and contracting then you can use a balloon and a cycle puncture pump. Using the pump you can make the balloon expand and contract.
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