Lactic acid
The result of laughing so hard your face turns red and your butt feels like it’s on fire from the inside out.
I laughed so hard at my friend’s terrible joke that my butt felt like it had a hot dog in it.
After my mom told me I was the worst kid ever, I laughed until my butt burned like it was on fire.
My cousin’s face when he saw my dog wearing a hat made me laugh so hard my butt felt like it was melting.