If your toilet seems to move when you sit on it and there's no apparent damage to the floor - or if it just feels crooked - chances are it's out of level. This is not only annoying, but it can make the flushing action of the toilet less efficient. Levelling a toilet takes just minutes with some plastic shims from your local hardware store. If your floor is ceramic, the project is particularly easy, because you can just caulk around the base afterward to hide the shims.
Lay your level at various points on the top of the toilet bowl (with the lid closed) to determine the slant's exact location. The side of the toilet that makes the bubble rise is the high side.
If the low side of the slant is on the left or right of the toilet, use your spanner to loosen whichever floor bolt is on that side. If the low side is at the front or back of the toilet, loosen both bolts.
Lift the low side of toilet by grabbing the rim of the bowl on that side and gently pulling upward. Slip three plastic shims under the base of the toilet on the low side, about 10 cm (four inches) from each other. Put them about halfway under the edge of the base, with the thick edges of the shims sticking out.
Set the toilet down on the shims and take another level reading. If it's not yet level, lift the toilet again and push the shims in another half a centimetre (1/5 inch.) If you've shimmed too high and the toilet is off level in the other direction, pull the shims out by the same amount. Continue adjusting until the toilet is level.
Tighten down the two bolts. Use your razor knife to cut off the excess parts of the shims that are peeking out from under the toilet. Lay a line of waterproof caulk around the toilet base on the floor to seal it.
If your toilet is more than half a centimetre (1/5 inch) or so out of level, you've got a bigger problem than shims can solve, and you should consider having a professional look at what might be done to the floor.