You, being the... not intelligent person you are, thought you didn't have to wear sunscreen when at the beach. How did that turn out? You got a really bad sunburn. Almost your entire body was bright red like a blushing schoolgirl. Sans told you to not be an idiot and put on sunscreen the entire time you were at the beach.

When Sans found out you got sunburnt... he had a field day, teleporting in your house and mocking you.

"Wow... Yer skin tone is a'pealing. I guess ya were always a well-red person!." He cackled at his own horrible puns. Lucky him. Being a skeleton and having no skin has it perks.