Falls; stair diving, picking fights with cars etc
2: Broken bones
Soft body meets fast moving hard object, or vice versa.
Sometimes this can be serious but breaking bones is the most common outcome of this pastime, that's not usually going to kill anyone but it might be a good idea to find out how to make a sling. The is really useful information if you happen to have a nice triagular bandage with you, if you don't you'll have to improvise.
In general if it's simply a broken arm or leg, then stop it moving it by tying an arm to the persons body or tying legs together, then get them off to casualty.
If an arm or leg is snapped tie it to a splint - a straight piece of wood or metal - to stop it bending and causing more damage. Again, tie legs together.
If you do this though, never straighten a broken bone, or swing a damaged joint - if the arm or leg is bent at a strange angle, do your best to keep it in this shape or position; bind as you find.
More serious is what happens if the fall is big enough, or they are hit by something heavy or fast enough, or if they're really crap at landing.
If this has happened, the person will be laying on the ground, perhaps unconscious or maybe groaning in pain.
Look for arms, legs and most importantly, heads, twisted at strange angles. A strangely twisted neck is extremely serious.
This is where you have to make a fast decision:
DON'T LET THE PERSON BE MOVED
If it later turns out not to be serious, no harm has been done by your caution but it's probably the most important thing you can do. If it is serious get help urgently.
If it doesn't look serious, still don't let them be moved until you have at least checked out a couple more things:
If the person can talk, ask them to move their feet and hands; can they feel you touching or pinching them? If not they stay where they are till professional help arrives.
If the person is unconscious or in pain - especially when they try and move themselves, then again they stay where they are. You may need help to keep them there sometimes.The reason for all this red text is they may have a neck or spine injury and you could easily make it a lot worse by moving them, they could be paralysed for life.
Just make them comfortable and get help.
The exception of course is if you have to get them away from some danger such as fire or a live wire, then be very careful how you move them.
If they're in a crashed car and there's no sign of fire, keep them in the car, leave it to the emergency services to get them out.