A toilet rim block is a substance in the shape of a block that is used in flush toilets, which slowly dissolves in water. The blocks usually come in a small holder that is attached over the rim of a toilet and hangs down into the bowl, so as the toilet gets flushed, the water passes through the holder coming into contact with the block. With "liquid rims", however, liquid is held in a small bottle above—and connected to—the holder that slowly releases into the bottom of the holder (which is beneath the toilet rim), and so coming into contact with the water when the toilet is flushed.[citation needed]
However, the blocks also come loose, for placement directly in-cistern (and therefore usable with squat toilets that lack the same sort of rim),[citation needed] although these tend to be slightly different in composition, so as to dissolve slower, due to the constant contact with water. These may also contain a colorant (typically blue or green) which shows up in the pan or bowl water.