Sometimes, getting a song stuck in your head can be a good thing, educational even. Thanks to Schoolhouse Rock! and an adorable scroll of paper, an entire generation can, four decades later, still remember how a bill becomes a law. Sing-along animated shorts like “I’m Just a Bill,” “Three Is a Magic Number” and “Conjunction Junction” taught kids about government, grammar and math. A new generation, Gen X, became hooked on the tunes thanks to the Schoolhouse Rock! Rocks tribute album, which was released 20 years ago. And subsequent generations keep getting hooked. The cartoons have been produced up through 2009, when “Earth Rock” brought eco-consciousness to 21st century children. Let’s take a look at some fascinating facts about Schoolhouse Rock!