The joke hinges on the misleading use of a homophone. Whilst the audience naturally assumes that the first nun is looking for the soap. Thus translating the homophone as 'Where's'. In fact the second nun is refering to how using the soap as a masturbation aid decreases the life of the bar through the increased deterioration caused by the frantic rubbing. Thus, the audience now re-interprets the homophone as 'Wears'.

I hope that clears things up.


Peace,

kowalski