Why should I prearrange my funeral or burial?

If I pre-plan and prepay my funeral or burial, how do I know that the money will be there when I die? 

If I prepay my funeral, what happens to the interest that my money earns? 

Can I change any of these prepaid funeral services later? 

What if your funeral home or cemetery changes ownership? 

Is there a pre-payment penalty for paying off these arrangements in advance? 

Can social services take my pre-arrangements away from me?