system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What are freckles and how do they form?
gpt
Freckles (also referred to as ephelides) tend to be little flat brown spots which usually are visible on sun-exposed skin, particularly the facial area and arms. Ephelis generally emerge throughout the warm calendar months.Freckles tend to be passed down and grow darker with steady exposure to the sun. Ultra-violet radiation from the sunlight stimulate pigment generating skin cells (melanocytes) to create pigment (melanin). Elevated melanin is demonstrated as darkish or liver spots on the skin.

Skin color depends on the amount of a pigment called Melanin it contains. The more Melanin you have the darker your skin color. Albinos have no Melanin at all in their skin! 

Melanin helps to protect you from the harmful ultraviolet rays in sunlight. When these rays hit your skin, the Melanin cells in your skin respond by releasing their chemicals which darken your skin and protect you better from the sun. 

In some people, the Melanin tends to "clump" in areas which appear darker than the skin surrounding them. This is how freckles come about. Melanin is regulated by the MC1R gene, and a variant of that gene is believed to determine how freckly you are. When you spend lots of time in the sun, these clumps of melanin are stimulated to produce more pigment thus you get darker, or even more freckles! The tendency to have freckles seems to be inherited.