You're flying over loose gravel like a bat out of hell and conquering that mountain... but jagged rocks and broken branches can give you a puncture quicker than you can say "off-road terrain," so you need to know how to fix a mountain bike flat tire before it has the chance to ruin your day.

Never fixed a puncture before? Here's a simple, step-by-step guide you can print out and take with you the next time you're on the trail.

First off, you need a puncture repair kit. It should include:

• Standard patch kit (glue, patches, sandpaper)
• Tire levers (or a dull screwdriver)
• Scissors (to cut the patches)
• Marker pen (so you can indicate hole position)
• Talcum powder

Step 1: Remove the wheel

If you're fixing a rear wheel, you need to remove the chain first.

1. Change into top gear so the chain is on the smallest gear.
   
   
2. Lift the chain off the top gear. It'll still be attached to the derailleur (which is attached to the frame), but won't be resting on any part of the wheel.

Then disconnect the brakes.

1. Unhook the brake cable from the cantilever.
   
   
2. Squeeze the brake cantilevers together to create some slack in the brake cable.
   
   
3. Use the slack to unhook the cable from whichever cantilever allows it. It'll still be attached to the other cantilever.
   
   
4. Grip the quick-release lever on the skewer to loosen the wheel.
   
   
5. Remove the wheel.

Step 2: Deflate air from the inner tube

1. You need to completely flatten the tire. Unscrew the valve nut, which will still be attached, but can now be depressed easily.
   
   
2. Push down on the tip of the valve to release the air until the tire is totally flat.

Step 3: Separate the tire from the rim

1. Start opposite the valve stem and carefully pry the tire from the rim using a tire lever (or an old dull screwdriver). Just be careful not to pinch or damage the inner tube!
   
   
2. Remove the tire from the rim by slowly rolling it. Make sure you don't damage the inner tube's valve stem as you're rolling.

Step 4: Mark the holes

Once you have the inner tube off you need to locate the hole(s).

1. If you can't see the holes immediately, pump some air into the tube and dunk it in water (look for where the bubbles are coming from. Alternately, you can hold the tube up to your ear and listen for the leak.
   
   
2. Mark where the punctures are with a pen or chalk.
   

Step 5: Sandpaper and glue

First check the overall condition of the inner tube. If it's already patched to death you might want to consider buying a new tube. Otherwise:

1. Cut a patch from your kit to the size you need.
   
   
2. Lightly roughen the patch area with sandpaper.
   
   
3. Wipe the surface.
   
   
4. Apply a thin layer of the glue to the tube. You will want to glue an area slightly larger than your patch.
   

Step 6: Patch it up

1. Wait until the glue is partially dry (tacky) then peel the backing paper from the patch and stick it over the hole.
   
   
2. Apply a little pressure on the patch for a few minutes.
   

Step 7: Replace the inner tube

1. Dust the inner tube with talcum powder (or chalk powder) once the patch has set to reattaching it easier.
   
   
2. Place the partially inflated tube onto the rim, with the valve inserted through the rim's valve hole.
   

Step 8: Fit the tire back on

1. Place the tire over the tube, tucking the edges of the tire into the rim on one side.
   
   
2. Add air until the tube is firm.
   
   
3. Secure the tire with a tire iron by levering the still-free edge of the tire into the rim so that the tire is completely inside the rim.
   
   
4. Be careful not to pinch the tube between the rim and the tire iron as it could rip or tear the tube.
   
   
5. Fully inflate the tube so that the tire is rock-hard.
   
   
6. Listen for any leaking air. You'll need to repatch if you hear any.

Step 9: Reinstall the tire

1. Replace the wheel onto the bike frame.
   
   
2. Replace the chain if your puncture was on the rear tire.
   
   
3. Lift the chain back onto the top gear.
   
   
4. Spin the pedals. The wheel should turn easily.
   
   
5. Hook the brake cable back onto the cantilever and squeeze the brake cantilevers together.
   
   
6. Use the slack in the cable to hook the end of the cable into the slot in the free cantilever so that the cable is attached to both cantilevers.
   
   
7. Spin the wheel. The wheel will turn freely.
   
   
8. Tighten the skewer by closing the quick-release lever.

Then you're done!

The most important thing to remember is to always have a puncture kit on hand to fix your mountain bike tires -- and a spare inner tube just in case you can't patch up your old one. You might even want to print these instructions and keep them with your kit for handy reference.