All boys do it.

If they tell you they don't, they're lying.

You know they're doing it when they sneak into the garage and hunt around in dark corners for things they know they're not supposed to play with.

Yep. They're doing it, alright.

Building obstacle courses.

Kids will spend timeless hours constructing elaborate, imaginative courses on which to test themselves: A plank of wood and two buckets become a rope bridge over a pit of hot lava; a swing set becomes jungle vines high above angry headhunters; an overturned folding table is the impenetrable wall of Attila the Hun's fortress.

When I see my kids building obstacle courses through the living room or across the front yard, I secretly yearn to join in. But it's just not the same. Through adult eyes, the magic has faded. The ability to willfully suspend disbelief and actually see the lava beneath you is all but gone.

So what are grown-ups with delusions of immaturity to do when they can't shake the craving to conquer the gauntlet?

Mud run!

Mud runs are what happen when those kids become adults looking for ways to realize and legitimize their childhood fantasies — and turn a profit while doing so.

If you're unfamiliar with mud runs, picture a military boot camp training course: pits of muddy water, barely scalable walls, cargo netting, high-stepping tire courses. Then imagine an entrepreneur setting up this torturous trek over a 5K course and charging civilians for the privilege of getting soaked, muddy and sweaty while running it.

People turn out in droves. And last week, my wife and I joined them.

It was awesome.

For those who like a little exercise, maybe even compete in local 5k/10k races, mud runs offer a fresh, entertaining and challenging way to compete. There's plenty of running, but throughout you're forced to overcome difficult obstacles.

The SoCal Rugged Maniac mud run (www.ruggedmaniac.com) in Temecula last week started with a short trot through a lake smelling of rotten eggs, just to make sure you knew your shoes were going to be wet the entire race. Then leap over a few walls before climbing up a mud hill and sliding down into a mud bog. Then another mud hill and another mud bog. And another mud hill and another mud bog.

Don't mind that guy in the water tanker. He's just trying to knock you off your feet with a water cannon. Then weave through a tire jungle before crawling through a 50-yard pit of muddy water. And you have to crawl if you don't want to be ripped to shreds by the barbed wire over your head. Climb another wall, then run uphill until your nose bleeds.

Mile One complete.

Are you getting the picture?

Climb a cargo net wall, then run downhill until your legs scream. Leap over three more walls, then crawl on hands and aching knees through a 75-yard tunnel. You're getting pretty hot now and those knees are really starting to burn, so it's time for a dunk. Slide down a four-story water “suicide” slide into (you guessed it) a pool of muddy water while trying to avoid the bodies piled up before and after you.