Medicare, designed to offer health insurance for those 65 or older, is clearly a big part of many Americans' lives -- but there are still lots of things about it that plenty of people don't know. See how much of this article surprises you.

When Medicare opened its doors in 1966, 19 million people soon signed up. Recent tallies show 37.8 million Americans enrolled in the original Medicare program, with another 18.1 million in Medicare Advantage and other plans, for a total of almost 56 million Americans in the system. Medicare, designed to offer health insurance for those 65 or older, is clearly a big part of many Americans' lives -- but there are still things about it that plenty of people don't know.