- Let them pick one card of three
- Offer to switch their card for the other two (closed!) cards. Most people take this deal, in my experience everyone takes this deal.
- Then, show the "goat"/useless card of the two remaining cards
- Offer them (again) to switch their original choice of card for the remaining card.
If they accept to switch, let them explain why (this is when people get it). If they don't switch, tell them that one of the two cards they could have switched for must have been a goat, so I could always show them one goat between these two.
After this, most people (all my test subjects, anyway) understand that in the long run, they're best off switching to the "two card" deal. They now understand that one must be a goat but that the probability of the "car" being among the two-card offer is twice as likely.
N<10 so YMMV