Any stone in a hiking boot migrates to the point of maximum pressure.

The distance to a given camp site remains constant as twilight approaches.

The number of mosquitoes at any given location is inversely proportional to the amount of repellent that remains.

The probability of diarrhea increases with the square of the thistle content of the local vegetation.

The area of level ground in the neighborhood tends to vanish as the need to make camp becomes finite.

In a mummy bag the urgency of ones need to urinate is inversely proportional to the amount of clothing worn. It is also inversely proportional to the temperature and the degree to which the mummy bag is completely zipped up.

Waterproof clothing isn't. (However, it is 100% effective at containing sweat).

The width of backpack straps decreases with the distance hiked. To compensate, the weight of the backpack increases.

Average temperature increases with the amount of clothing brought.

Tent stakes come only in the quantity "N-1" where N is the number of stakes necessary to stake down a tent.

Propane/butane tanks that are full when they are packed, will unexplainably empty themselves before you can reach the campsite.

Given a chance, matches will find a way to get wet.

Your side of the tent is the side that leaks.

All foods assume a uniform taste, texture, and color when freeze-dried.

Divide the number of servings by two when reading the directions for reconstituting anything freeze-dried.

When reading the instructions of a pump-activated water filter, "hour" should be substituted for "minute" when reading the average quarts filtered per minute.

The weight in a backpack can never remain uniformly distributed.

All tree branches in a forest grow outward from their respective trunks at exactly the height of your nose. If you are male, tree branches will also grow at groin height.

You will lose the little toothpick in your Swiss Army knife as soon as you open the box.

Rain. 

Enough dirt will get tracked into the tent on the first day out, that you can grow the food you need for the rest of the trip in rows between sleeping bags.

When camping in late fall or winter, your underwear will stay at approximately 35.702 degrees Kelvin no matter how long you keep it in your sleeping bag with you.

Bears. 

The sun sets three-and-a-half times faster than normal when you're trying to set up camp.

Tents never come apart as easily when you're leaving a site as when you're trying to get them set up in the first place.

When planning to take time off of work/school for your camping trip, always add an extra week, because when you get home from your "vacation" you'll be too tired to go back for a week after.