Here is a piece of good news hiding inside every song ever written: melodies are not made of hundreds of different things. They are made of about a dozen kinds of jump, reused endlessly. Learn to recognise the jumps, and unfamiliar melodies stop being a blur — they become spelling, with an alphabet your ear already half knows.
What an interval is
Think back to the siren from Module 1. You slid your voice through every possible pitch, like running a finger along a ruler. Intervals are the marked measurements on that ruler. "Happy Birthday" begins with the same sizes of jump whether you sing it low, high, tired or delighted — the starting note can change, but the distances are the song.
That is why you can recognise a tune whistled by a stranger in a completely different key: your brain stores melodies as chains of intervals, not as fixed notes. Your ear has been doing interval maths your whole life. This lesson simply introduces that skill to your voice.