I can give you the answer in one word: evaporation.

Now for the longer answer. Evaporation is the process where liquid water turns into water vapor. (Look at the word E-vapor-ation.) When water evaporates, all of the stuff thats dissolved in the water stays behind.

Water dissolves all sorts of things, so even clear water usually contains things like salt, calcium, and other things in very small amounts that we cant even taste. If your family uses a tea kettle or coffee maker, you can see the white crystals that are left behind after weeks of heating water. The water itself evaporates, but the things that were dissolved in it stay behind. Water does not have to boil to evaporate, but evaporation happens faster at higher temperatures.

Places like the Great Salt Lake of Utah and the Dead Sea get water from rivers, but there are no rivers going out because they are surrounded by higher ground. Water can only leave by evaporation. Even fresh water has a little salt in it, so after millions of years, quite a lot of salt builds up in these places. You can do your own experiment with the help of an adult. Warm up some water and dissolve salt in it. Store it in a closed container. Pour a little in a bowl and let it sit somewhere where it wont be disturbed. As it dries up, add more of your salty water. After a week or so, the water in the bowl will be saltier than the water in the closed container. How long this takes depends on how much evaporation is happening. The Dead Sea and the Great Salt Lake are in very hot, dry places where theres a lot of evaporation.

People float really well in the Great Salt Lake and the Dead Sea. Can you figure out why? Ill give you a clue in one word: Density.