...when you make a joke to your best friend who also has PTSD and she snorts when she laughs, causing you to laugh and you laugh so hard that your brain switches off and you can't remember why you're laughing so you ask your friend who repeats the joke which you find hilarious, having never heard it before, and you laugh hysterically causing her to laugh hysterically until you laugh so hard that you switch off again and forget why you were laughing, repeating this three times until you have a cluster headache, can't move and are still laughing about something that you can't remember.