The blocks are used to kill germs and take away the bad smell of urinals in public bathrooms. They are put over the urinal drain and evaporate very slowly. Because they do not dissolve or break apart in water, they do not take away the smell of the flushing water or urine. Different blocks are made with different chemicals, like paradichlorobenzene or naphthalene. The block is something scented to smell like different things, such as cherries.