When a fish swims through water, it experiences drag caused by fluid friction. To swim successfully, a fish must use its strength to overcome resistance caused by friction. In clear waters, the water is quite thin, making swimming easier, but in murky waters, fish experience more friction, making swimming difficult.
In addition to liquids, the term fluid friction may also be used to describe friction caused by gases, such as oxygen and nitrogen. For example, planes that fly through air also have to fight fluid friction. This is why aerodynamic research involving studies and experiments regarding how objects move through air is so important.