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Make a Sponge Fossil

Have you ever wondered how bones become hardened fossils? Try this Real Science activity to find out!


  • A small kitchen sponge
  • Scissors
  • A pen
  • A large bowl of very salty water
  • A bucket half-full of sand


  1. Cut the sponge into the shape of a bone.
  2. Soak the sponge bone in the salty water.
  3. Bury the sponge bone in the bucket of sand.
  4. Pour the rest of the salt water on top.
  5. Let the water evaporate for a week or so.
  6. Dig up the sponge and see what happened.

What's going on:

The sponge should be hard because the salt crystals have filled in the spaces. This is similar to fossilization. When a real bone is buried in the ground, minerals seep into the spaces inside, water evaporates and the bone becomes hard. This process, called permineralization, is one of several similar processes that create fossils over time.
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