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Cut a sponge so it fits in the bottom of the dish, container, or lid. Put the sponge in the bottom of the dish. Next boil water, remove from heat, and add the salt a little at a time. Stir well after each addition of salt. Next add vinegar and stir well. Pour the salt and vinegar mixture carefully over the sponge, just enough to cover the bottom of the dish. Pour the extra mixture into a container and save. Put the dish in a safe, sunny place. After a few days, salt crystals will start to form. If the salt and vinegar mixture evaporates, add more.
Why does this happen? Salt is a mineral. You boil the water because you can dissolve more salt in hot water than in cold water, making the water overloaded with salt. The sponge soaks up the salt and vinegar mixture. As the solution evaporates, salt crystals are left behind on the sponge. Look at the shape of your salt crystals. What are they?