Trap Function

• Just below the toilet's bowl, at the start of the toilet's drain line, is a curving pipe called the trap. You can see the trap from the side of some toilets. The trap stays full of water, if the toilet is functioning normally, and keeps the water level in the bowl the same. When you flush the toilet and more water flows out of the tank and into the bowl, the water spills over the top of the trap and flows down the drain into the pipe in the floor.
  
  

Siphoning

• You toilet bowl's water level may drop slowly over time from siphoning caused by a partial clog of toilet paper. If the toilet paper clog spans the top of the trap, the absorbent properties of the toilet paper can draw water out of the trap and down the toilet's drain line. As the water empties out of the toilet's trap, the bowl level drops as well.
  
  

Cracks

• The toilet's bowl is made of porcelain, which can crack easily. Pouring hot water in the bowl, hitting the bowl or dropping heavy or sharp objects against the inside or outside of the toilet's bowl will cause the porcelain to crack. You can also crack the bowl by over-tightening the nuts on the bolts on the toilet's base. If the cracks sit on the trap or bowl, the leaking water will drop the bowl's water level.
  
  

Clogs

• A clog can elevate the water level in the bowl since the clog will not allow the water coming in from the tank to spill over the top of the trap like it normally would. If you have a clog, the first thing you should do is press a plunger over the drain hole in the bowl and pump the handle up and down a dozen times or more. If the clog still holds up, feed a toilet auger down its drain line as you spin it clockwise. Serious clogs that you cannot remove after using the auger three times in the drain require the expertise of a plumber.