How did I grow to (the age of) adulthood without learning this one?
While racing your engine or just driving down the highway, turn the ignition off then on again. You'll hear one or more loud explosions. Bystanders will think they're being shot at. Possibly your muffler will be destroyed.
This works with gasoline engines.

How it works:
When you turn the key off, the sparkplugs no longer fire. So that perfect gas-air mixture your car was running on gets pumped into your exhaust system. Then when you turn the key back on the sparkplugs fire and the hot exhaust detonates the rich mixture in your exhaust system like a huge potato cannon.

I did it with an old pickup with no catalytic converter. With a catalytic converter things might be different, but you know what? I'm not going to test it.
If your car has been running for long enough to heat up the catalytic converter it will probably burn the fuel-air mix before too much builds up. If the converter is cold it might act as a spark arrestor. Either way might result in a smaller volume detonating at once. And any damage is likely to be more expensive.

This fun new trick has cost me $90 so far.

Step 1: Step Two: Get a New Muffler

My mom and I are driving from Minnesota to California in the best little truck ever built, a '78 Toyota with the 20R engine and a manual transmission. Right now we're in South Dakota.
My mom was driving. Her key for the truck isn't well cut so it tends to jam in the "start" position, which will burn out the starter. I heard a funny noise and wanted to make sure the starter wasn't grinding itself to death while the motor ran, so I turned the key back a notch, which turned off the spark, then back on.
KABANG! the truck jumped a bit and now was very loud with no muffler.
After checking the damage we kept going with earplugs in and switched to sign language. "Great Sunset"
and "There ought to be more boats on that river" are pretty easy to sign, even if you have to invent the language from scratch.

I would have beat it back into shape and welded it with coat hangers, but time is short and I don't get to see my mom that much, so I checked with the garage at the truck stop when we stopped for the night in Murdo, South Dakota.
Fortunately they had the right replacement muffler, so for $90 I got the thing replaced at 10pm on a sunday night. Wow. Thanks Cole Venard and Bob at the Triple H Truck Stop! (605)669-2465

I'll give the guys a break while they fix it and wander around the shop for a bit.

Step 2: Gawk at Truckstop Stuff Like Truck Testicles

I've been asleep for a hundred years, so excuse me if I don't already have a pair of giant rubber testicles hanging under my truck's bumper. These are a huge seller.

This sticker collection is really good too. I especially like "Hi. Eat me." and "You suck and that's sad". These are reusable stickers so you can put them on and off your windows as the mood suits.

For a dollar and thirty cents I bought a see-through lighter with a built-in flashlight and a compass that really works. Damn! Technology doesn't stop EVER!!!

Step 3: Wooden Block Ramps and a Nifty Chair

Back to the garage.
This chair is comfortable. The seat is made from the top of some kind of tank.

Here's a good use for all those chunks of studwood you've been hoarding. Make a pair of ramps. These raise the vehicle 4", just enough to get under it with a creeper, not enough to make it dangerous.

I'm sorry to leave this truckstop. Everyone is friendly and has good helpful things to say. I could have gotten a stainless steel muffler even. Maybe stayed here forever.

But in a few short weeks it's going to be a million degrees below zero here and blowing a hundred miles an hour. And now it's midnight and I better get to sleep or I'll miss Mt.Rushmore.
Good Night America!

Step 4: $100 Off Your Next Windshield, Cure for Grinding Starters.

In step 1 of this instructable is the story of how we drove the truck a long way with the starter on.
That was bad and damaged the gears of the starter, probably the flywheel as well.
Now when I turn the key there's usually just a grinding sound, no starting.

That's okay because this truck has a manual transmission and can be push started.
"Only own cars with manual transmissions" is the the 11th Commandment.
Actually, let's make that Commandment #1 and push the other Commandments back.
That's how important it is to having a happy life.

My mom is a good sport. Every time we push started the car she broke out laughing. A couple of times gangs of motorcyclists or tattoed youths with bad hair helped push the car.
Push-starting your car enables you to see people at their best.
It always makes people happy to see you run along your car pushing it, jump in, and have the engine fire right up when you pop the clutch in 2nd gear.

I thought I'd have to replace the starter and maybe the flywheel. I jacked it up a couple of times and crawled under it but lost enthusiasm and crawled back out.

So I had the cracked windshield replaced first. "I don't have insurance. How much to replace my windshield?" Those magic words will save you $100. Insurance is a hassle for everyone, and people don't shop for price for insurance jobs. Thus they charge you less if you say you don't have it. Try it. it's true. It works for me anyway.

Then I procrastinated more by paying my taxes, fixing and cleaning a toilet, got a couple more free boats, etc. etc. and finally it was time to fix the starter. Fortunately my pal Joe Van Niekerk from South Africa came sailing with me. He taught me a trick for starting a car with bad teeth. When you park your car, stop the motor by stepping on the brake, putting the car in gear, and letting out the clutch. That stalls the motor and stops it right away. That way the motor stops in a position where there are fresh teeth.
It works like a charm. After stopping the motor that way, the starter works perfectly every time because the flywheel teeth are fresh and sharp in that position and the starter gear engages well.
When you do it the usual way and stop the motor by turning off the ignition key it stops in the same old positions where the teeth are already shot, so the starter just grinds and doesn't mesh with them.

So there you have it.
Stall your motor and spare your starter.
And the magic words that save you $100 on your next windshield. Or any other bodywork for that matter.