Fred pointed out this Althouse post, which retells an old joke about Libertarians:

If you want to find out if someone's really a libertarian, ask him: Do you think children should be allowed to buy heroin from vending machines? A real libertarian will answer: Only if the vending machines are privately owned.

Libertarian principle is based on individual liberty, between consenting adults.  If you say that Libertarianism is about individual liberty, and forget the second part, then murder would be perfectly OK, since I would have the individual liberty to shoot you in the head.  Libertarianism is based on the idea that if I consent to something, and am willing to accept the consequences, then government's only role should be to mediate a dispute that arises when one person does not keep up their end of the bargain.  That's why contract law has always been so important, and why government's willingness to be a neutral arbiter in a contract dispute is key.  That's why the recent push by the federal government to intervene in sub-prime mortgages is such a mistake for instance.  They're rewriting the contracts after the fact, and the mortgage companies will have no faith in the future that future contracts they make with anyone will be honored.  This will have dire consequences.

But back to the joke.  Libertarians wouldn't say it's OK for children to buy heroin from a vending machine, because children are not consenting adults.  They don't have the ability to make that choice, and accept the potential consequences.  If you don't have the latter, then you can't have the former.  But of course, being allowed to do something, and still being able to do it are two entirely separate matters.  Adults aren't allowed to buy heroin today at all, and yet they amazingly still have the ability to do so at will.  It's a stupid joke.

Where did the child get the money to use in the vending machine in the first place?  Where was mom and dad while he was shooting up?  When did it become the government's responsibility to watch over your child.  Why did you abdicate that role?  If government now has the responsibility to keep your child from buying heroin out of a vending machine, then why shouldn't it also make sure that your child doesn't buy Twinkies and get fat?  And if government is going to do that, then why shouldn't it also make sure that your child has the correct education that the government chooses, even if it means teaching evolution without the guiding hand of God?  Oh... that's taking it too far now isn't it?

But if I might respond with a joke of my own.

"If you want to find out if someone's really a Republican, ask him: How did your child get the money to buy heroin from a vending machine?  A real Republican will answer: I gave him the money and sent him to the store to buy a Bible."

*Ba da da, ching*