system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How sunburns occur, and why they can cause pain for days after receiving the sunburn.

The only thing I know about the burns is that the UV rays cause the burn.

 I have no understanding of how the pain occurs, or why it lasts days after getting the initial burn. On top of that, why (when the burn is bad enough) your skin may secrete/drain(?) fluid from the burned area.
gpt
When you get a burn, what you are doing is causing damage to the DNA in your skin cells.  There is a chemical called thymine that is one of the bases in your DNA.  When it is hit by UV light, it can get knocked out of place and attach to a thymine next to it.  This forms what is called a dimer.  

When many dimers form in your DNA, your body knows something is wrong and starts the process of healing.  Several signals are sent using proteins.  Among these is a trigger for the area to get increased blood flow (resulting in redness and warmth) and increased sensitivity to pain.  

Intense sunburns can cause blistering as the living tissue underneath the skin dies from heavy DNA damage.  This causes layers to separate and the empty space to fill with fluid.