Acrylic paint is mainly made of color pigments, and binding agents, and occasionally others contain added agents, which provide properties like durability.
This is the most popular, easiest, and cheapest way to thin acrylic paint. The most common ratio to use is 50% water to your paint, but some say 30% is sufficient.
Using an acrylic painting medium like an airbrush medium might be a better solution. The airbrush medium has a consistency similar to water and is easy to use.
Pouring medium was designed to help with improving the flow of acrylic paint. The medium is used mostly in the now popular art form of acrylic pouring.