Seconds before impact...

I think you can guess what happened next.  Who needs stopping distance when you have an oblivious big brother to crash into?

Well, turnabout is fair play.  At least River is bracing for impact.  I'm not sure if this is an example of really poor depth perception or if they're doing it on purpose.

I take that last part back...they are doing it on purpose!
This time, they both started zooming around the apartment and once they reached ramming speed, they actually head-butted one another like a pair of mountain goats you see in those wildlife shows.
I was stunned (and so were they).  I ran over to them to make sure they both weren't concussed.  Thankfully, they mellowed out after that and decided that noogies and cheek rubs were just as satisfying as near-death experiences.