As water flows across rocks and soil in rivers, it picks up trace amounts of minerals and salts. The rivers then flow out into the ocean along with these trace elements. Ocean water does not flow anywhere and only evaporates. When the ocean water evaporates, the salt and minerals are left behind. As this process repeats itself over and over in time (water flowing in, water evaporating out) the concentration of salt becomes greater. The amount of salt in the ocean varies, but is on average 3.5 percent by weight.