Most embarrassing thing ever?

Anyways i just like to get your thoughts about this. I was on the trampoline with my sister and sisters friend. I'm 15 and they're both 9. My sister went to go get water for the three of us and thats when my sisters friend asked me to do a flip. I thought what the heck i know how to do one but write as i did my pants flew off. I wasn't wearing boxers so my penis was out in the open and she had full view. She covered her eyes and started laughing and i quickly put my pants back on and left feeling like weird. Any thoughts on what i should say?