Vermont Sen. Bernie Sanders, the independent socialist who's running for president, plans to introduce a bill to make college tuition free at public colleges and universities.

Details on how the Sanders plan would work are sparse; he's not expected to introduce the actual legislation until Tuesday. And while free tuition might be hard to imagine in the United States, when you look abroad, it's not just a pipe dream: many European countries have these types of plans in place right now.

Germany made headlines in the fall when it abolished tuition fees for its public colleges. Tuition is cheaper in Canada than in the US. "Countries like Germany, Denmark, Sweden and many more are providing free or inexpensive higher education for their young people," Sanders said in a statement about the forthcoming plan. "They understand how important it is to be investing in their youth. We should be doing the same."

How other countries offer a much cheaper higher education

There are a few reasons higher education is cheaper in the rest of the world. It's often more centralized, which gives the government more authority to set prices. Higher tax rates often pay for more state spending on higher education. And the system itself works differently, channeling more students into non-college options after high school and spending less on campus culture and other amenities.

Denmark's system, among the most generous in the world, is financed by much higher tax rates than in the US. In Sweden, tuition is also free, but students take on a surprising amount of debt — about $19,000 per person in order to pay for living expenses. Germany's colleges and universities are free, but as Rebecca Schuman wrote at Slate, but they're also less student-oriented. It's much more common for students to commute, and students are expected to declare a major right away, which presumably cuts down on administrative and advising costs but also restricts students' choices.

Many countries with free or much cheaper tuition still have student loan programs to help defray the non-tuition expenses that come along with higher education (eating, renting an apartment, buying books, and so on). Denmark, for example, pays students about $900 per month just to go to college.

Could Sanders bring free tuition to the US?

Most countries with free tuition have higher tax rates than the US, and in some cases a much more centrally controlled higher education system. That makes it easier for the national government to dictate higher education policy.

It's not clear how Sanders would get around that, but there are a few ideas out there. Obama's free community college proposal was really a state program that would require states to eliminate community college tuition in order to participate, and in exchange would cover much of the cost.

Another idea came from Jordan Weissmann, now at Slate: making higher education more like the K-12 system, where the federal money provides money directly to schools. Instead of offering grants, loans, and tax credits to individual students to use at whatever college they want, the government could provide that money directly to public higher education systems. It would be enough to cover the amount students are currently paying in tuition at public colleges and universities, but at the expense of aid to attend any other type of college.

And that means this is extremely unlikely to happen. Private colleges are a powerful lobbying contingent on Capitol Hill, in part because they're ubiquitous — just about every congressional district has a college in it — and in part because they're well-organized. They'd naturally oppose any attempt to cut off their students' grants and preferable loan terms.

But as the Obama administration learned before the State of the Union address this year, the promise of "free college" has a magic that reasonable student loan interest rates and flexible repayment plans do not. Eligibility for Tennessee's free community college program has far outpaced expectations. Student loan interest rates used to be a potent political issue for Democrats. They're discovering that not having to borrow to pay tuition at all is a much more powerful one.