A Buckle
A buckle is when you laugh so hard your knees give out, like you're watching a kid fall off a slide and it's the funniest thing ever.
A buckle happens when your friend tells a joke about a chicken and you can't stop laughing.
A buckle happens when you see a kid fall into a puddle and you start laughing so hard you fall over.
A buckle happens when your teacher says 'I hate your life' and you can't breathe from laughing.