Bladder

Bladder is one of the six needs your Sims have on the Needs bar. It is represented by a toilet. There is no other way to lower this need; it will gradually lower by itself.

You can raise this need by simply going and using the toilet. Nothing can affect this need. You can not raise your Sim babies' bladder bar, you have to wait for the baby to wet them selves.

For adults if you don't put them on the toilet they wet themselves and it takes 3 seconds to clean up.