We have a pretty good hand-me-down bike system in our household. A lot of my old mountain bike parts find their way on to other bikes, and as the kids grow they get the next larger bike hanging in the garage. Today, Eli inherited the bike that Hanna originally got in 2008. It's a 24" wheel mountain bike-ish object. Both Hanna and Tyson have pretty much surpassed it in size, and Eli just fits.
I cut the seat post down (an old Bontrager carbon post from my old Superfly) and lowered the FSA carbon handlebars (from my 2006 Cannondale Rush) and he was rockin'.
He was marveling at how easy it was to go uphill. Yeah, gears will do that for you.
Deanna and I took him to Cliff Cave and I took him on almost every trail. I was so surprised at how well he got down the trail. He was so excited! I could not believe how long we rode...probably an hour? Deanna was walking Rex while Eli and I were riding. I had to go back into the trail to find them...it gets a little confusing back in those tight trails. But we all came out alive and Eli had a brand new adventure that I hope lasts a lifetime.