Quite simply, the salt in the ocean comes from rocks on land. But the process is slightly more complicated than that. Rain that falls on land contains some dissolved carbon dioxide (the stuff we humans breath out after taking the oxygen we need out of the air). When carbon dioxide dissolves in water it forms carbonic acid, which, like its name, is slightly acidic. The acid in the rainwater then causes rocks to slowly erode and break down. When the rocks break down from the rainwater, ions, electrically charged atomic particles, are formed. So where do the ions go? They get carried away in the runoff to streams and rivers and, eventually, they make it all the way to the ocean. Some organisms (plants and animals) in the ocean use the ions for energy and other processes, but some of the ions end up left over and over time the number of ions in the ocean builds up. Ions come in many different shapes and sizes and forms and from many different kinds of rocks, but two of the most common ions that end up in the ocean are chloride and sodium. In fact, sodium and chloride ions make up over 90 percent of all dissolved ions in the ocean! Now if you have ever played with magnets you will know that the positively charged end of the magnet (the one with a `+´ on it) will stick to and be magnetically attracted to the negative end of another magnet (the one with the `-` on it). Sodium is negatively charged and chloride is positively charged and so, just like those magnets, they are magnetically attracted and they stick together. When they are stuck together they become sodium chloride, the chemical term for what we call salt! And there´s a lot of salt in the ocean! According to the National Oceanographic and Atmospheric Administration (NOAA), if the salt in the ocean could be removed and spread evenly over the Earth´s land surface it would form a layer for than 500 feet thick (or about the height of a 40 story building)!