Salinity Lab

In this lab we tested the salinity of different water samples looking for salinity and density. To do this, we measured the density with a hydrometer, the salinity with a temperature density device. The more salinity the denser the water sample. The marine and unknown samples had higher density than the brackish, tap ad lake samples. Because of this they also had a higher salinity. The temperatures of the samples varied, but the average temp. was about 20 degrees. I was interested to see that even the tap and lake sample had high salinity reasons because I just assumed "fresh" water would not have salt in it. After the lab I was still left wondering... how does the ocean become so salty?