My question is... How? Don't you need to have something to sue for? Medical bills and time out of work come to mind. But if you slip and fall and simply bruise your butt and go back to normal walking within 10 minutes.. What do you sue for?

You could jump out from behind a box and scare me, never even touching me, and I could sue you for something like "emotional trauma", or something. It would be my responsibility to argue to the judge that your actions did have a severe negative impact on my life, and thus it is reasonable for me to be compensated for them.

Basically you can sue someone for any sort of impact they have on your life, whether emotional, physical, or mental, granted, you can argue that what you're suing for is reasonable. Whether or not the judge agrees totally depends on if the judge thinks it is reasonable.

The emotional/mental aspect becomes a lot easier to sue for if there is a physical aspect accompanying it. Say you punch me in the face and my medical bills are only $1000, however, the event traumatized me so much gave me some sort of crippling anxiety that ruined my career. If I got a therapist to agree to that point of view, then I could sue you for a whole lot more than $1000, based on the potential damage you did to my career and my overall trajectory in life.