Cardspace, SAML, and OpenID go into a bar.
The bartender says to Cardspace 'I'm gonna have to see some ID before I serve you'. Cardspace shows its ID and then orders a beer.
Bartender says the same thing to SAML, which also orders a beer.
When the bartender sees OpenID, he says 'Im sorry but you are going to have to leave'.
'But why' says OpenID 'Im of legal drinking age and I have ID to prove it.'
'Im sure you do' says the bartender, 'but I don't know how to ask you for it'.