when you cracking up it means you laughing so hard at stupid and retarded shit people say or be doing....and most of the time people is hating you so badly and want to sabotage everything you do and be so evil to you because you so far out their league they cant stand it. and do everything to bring you down and always end up in prison forever or dead the worst way possible like being burn alive.
Means when, you say a joke so funny that even you have to laugh, even if you are the only one. Though it's kinda lame laughing with yourself, but, you just cant help yourself :)
haha, knock knock, whos there? boo..boo who?
ohh dont cry its only a joke...HAHAHA