You shouldn't steal from customers.
Mary is working as a waitress in a restaurant and notices a guest left her wallet on the table before leaving the restaurant.
Mary wants to remove the wallet from the restaurant table.
Mary quickly runs out of the restaurant and hands the guest her wallet.
Mary picks up the wallet and pockets the money that was inside, before throwing the wallet away.
Mary's guest is happy she can now spend the money instead of having to go look for the wallet.
Mary's friend goes without that money for the rest of her life, which makes her temporarily upset.