Q : why our skin can become darker if we get burn by the sun??

A : because, sun light contains ultraviolet which is bad for our body. as you know, that people's skin contains brown pigment (melanin) which absorbs ultraviolet so that we won't have greater ultraviolet in our body. when we have sunbathe, our pigment become more in amount to protect our skin. in human, for those who  live in warm area, their skin contains  more melanin. this is the product from area adaptation.

Mini info : fleck is a black stain which is created from melanin pigment