This intrepid Saturday Mechanic isn’t taking any chances here. He’s got the car up on stands, the wheel is under the car, and he’s wearing a mask and safety glasses.

Let’s say it’s time to replace the shocks. The job really isn’t that tough or inherently dangerous. But it does involve getting your car up in the air so you can work underneath it. And that’s where things can get troublesome–quickly. Your nifty new floor jack makes short work of putting another foot of daylight between the bottom of the car and your driveway. And two short stacks of stout cement blocks will be amply strong to hold it there. The car will be plenty stable, because those blocks have a wide footprint. Right?

You’ve already broken the lug nuts loose while the car was still on terra firma and removed the wheels. So you casually close the door to turn off that annoying buzzer. That sends a vibration through the chassis and causes a big, big problem.

In the blink of an eye, the front end slides right off the jack and every single one of those four cement blocks returns to its sand-and-gravel ancestry, leaving your poor car perched on four naked brake discs.

Imagine, just imagine, if you were under the car, wrench in hand, when this happened. Ouch!

Lessons Learned

Working on your own car can be easy and fun, but it’s got some potential dangers if you don’t use common sense. The most obvious example we just played out: Wrenching underneath a car is a good way to become two-dimensional if you don’t take the proper precautions. Let’s catalog some procedures.

The only appropriate place to jack up a car is on pavement. And in our case pavement means concrete, not softer asphalt. A jack stand can make a nice cookie-cutter hole in thin asphalt. And that’s especially true on a hot day, when the sun has made asphalt the consistency of molasses. Speaking of stands–always use `em, folks. Concrete block is not acceptable, because it’s far too frangible. There are really only three options: ramps, old-school jack stands and, of course, a hydraulic lift. Ramps are great if you just need to change the oil. But for suspension or brake work, you’ll need to remove the wheels and get into the wheel well. That means jack stands. They’re not expensive, so splurge and get a pair that’s rated for your largest vehicle.

What about wood–say that big stump over in the corner of the yard? Again, it’s possible for wood to crack and separate under stress. I recommend wood to protect the bottom of a sheetmetal pinch weld or chassis component while a vehicle is being lifted. And wood is good for other stuff too. Scrap 2 x 4s are certainly fine for blocking wheels to keep the car from rolling off the stands. That reminds me: Don’t trust the parking brake or the parking pawl in the transmission when you’re working under a vehicle. A friend of mine learned that the hard way one day while changing the U-joints on his pickup. He’d left the truck in park and didn’t bother to block the wheels. As he removed the last bolt from the differential flange, the truck rolled right over him, down the driveway and into the ditch across the street. Then he had to crawl under the truck and, lying in muddy water, reinstall the driveshaft so he could drive out of the ditch. Embarrassing, yes. But potentially deadly, too, if his truck hadn’t been so tall it rolled completely over him. Not to mention the truck rolling into traffic.

It’s just a quick fix, you say, so why not use a floor jack? No sir. A floor jack is not safe to support a car by itself. The hydraulic pressure inside the jack’s slave cylinder is what’s holding up that car. If any one of those rubber seals inside fails, the jack can dump pressure in a real hurry. And let’s not even consider the possibility that Saturday Mechanic Jr. might wander into the garage and try to “help” Daddy by twisting the jack handle. It’s happened, dude. Trust me.

Read more: 6 Ways to Stay Safe in Your Garage – Popular Mechanics