How do you hold in a laugh without it hurting

So I was at school and out of no where a girl farted so hard and I am immature and so it was hard not to laugh and I almost passed out so how do you hold in a laugh without it hurting

Sorry, I laughed just reading this! Sadly I don't think it's humanly possible to laugh without making any sounds, although I try all the time... Laughing is good for you though, and someone farting is a great time to laugh. Unless it's your best friend. In that case, laugh as hard as possible and THEN help them out. ;D