Have you ever noticed that, in the summer, when your sweat dries it will leave marks on your T-shirt? The marks are tiny bits of salt.
If you go to the seaside, you will know that seawater does the same to your clothes, because it also has a lot of salt in it.
Water evaporates from the surface of the sea but the salt remains. Have you ever thought about whether the seas will keep getting saltier? The answer is no. The oceans have stayed at about 3.5 per cent salt content for hundreds of millions years. It maintains a constant level of salt in different ways.
Pick up a clamshell and weigh it in your hand. It is heavy. All creatures need sodium to live and most need calcium to build bones and shells. The clam, like all sea creatures, gets its sodium and calcium from seawater. When creatures die, their salt is locked up in bones and shells, which drop to the bottom of the sea.
Reactions between seawater and rocks are not just one way. Sea salt not only dissolves from rocks, it also reacts with the rocks of the ocean crust and volcanic lava. The reactions take some of the salts in seawater away from the sea.
The outer hard crust of Earth is made up of a dozen or so hard plates that drift on extremely hot, soft rock like floating islands on a sea. The heat within Earth is not the same everywhere and the plates move due to the heat.
When an ocean plate bumps into a land plate, the less dense land plate floats over the ocean one. The ocean floor gets pushed under, and its salty rocks, along with shells and bones, are lost deep within Earth.
So, that's why the seas are salty but don't get any saltier.
evaporate v. 蒸发
clamshell n. 蛤壳
sodium n. 钠
clam n. 蛤
dissolve v. 溶解
react v. 起反应
crust n. 外壳
lava n. 熔岩
drift v. 漂流
dense adj. 密集的
（来源：21世纪报英语教学网 英语点津 Annabel 编辑）