**Introduction**

The salinity of seawater refers to the concentration of dissolved salts, including sodium chloride (common table salt), in the water. The average salinity of seawater is about 3.5%, which means that for every 1,000 grams of seawater, approximately 35 grams are dissolved salts. The process through which seawater becomes salty can be explained by a combination of geological, chemical, and biological factors.

**Geological Factors**

1. **Weathering and Erosion**: The Earth's crust is composed of various minerals, including salts. Over time, weathering and erosion processes break down these minerals and release them into rivers and streams.

2. **River Discharge**: Rivers carry the eroded mineral particles, including salts, into the oceans. The amount of salts contributed by rivers to the oceans is significant, and it has been estimated that about 4 billion tons of salt are delivered to the oceans every year.

**Chemical Factors**

1. **Evaporation**: As seawater is exposed to the Sun's heat, evaporation occurs, causing the water molecules to vaporize and leave behind the dissolved salts. The remaining water becomes saltier as the concentration of salts increases.

2. **Sea Ice Formation**: When seawater freezes to form sea ice, the salt is excluded from the ice crystals. This process further increases the salinity of the surrounding seawater.

**Biological Factors**

1. **Marine Organisms**: Numerous marine organisms, such as phytoplankton, algae, and bacteria, play a role in the salinity of seawater. These organisms extract dissolved salts from the water for various biological processes, and when they die or excrete waste, the salts are returned to the seawater.

2. **Accumulation of Dead Organisms**: Over long periods, the remains of marine organisms, including shells and skeletons, accumulate on the ocean floor. These deposits, known as sedimentary rocks, contain significant amounts of salts. Through geological processes like uplift and erosion, these rocks can release the salts back into the seawater.

**Conclusion**

The salinity of seawater is a result of a combination of geological, chemical, and biological factors. Weathering and erosion contribute salts to the oceans, while evaporation and sea ice formation increase the salt concentration. Marine organisms play a role in the extraction and release of salts, and the accumulation and subsequent release of dead organisms also affect seawater salinity. The interactions between these factors have resulted in the current salinity levels of seawater, making it essential for the survival of many marine organisms and influencing various oceanic processes.