Sea Monkeys are really brine shrimp, and brine shrimp are an incredibly cool life form. Brine shrimp are naturally found in salt lakes, like the Great Salt Lake in Utah. In a salt lake like this, the water is so salty that the only things living in it are the brine shrimp and certain types of algae on which the brine shrimp feed.
One of the things that make brine shrimp fascinating is their ability to lay encapsulated eggs called cysts. A cyst can dry out and remain viable for years. If you put brine shrimp cysts in salt water, they hatch very quickly, and the shrimp mature in about eight days. They can grow to be pretty big -- about half an inch (15 mm) long.
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