Major second

Okay kiddo, so you know how you can sing a song that goes "Twinkle, twinkle, little star" and it sounds good, right? Well sometimes when people make music they use different notes that are close to each other, but not too close. And one of those notes sounds like "Twinkle, twinkle" but starting from the second "twinkle". That's called a "major second".

It's like if you had a toy piano with different colored keys. You might play the white key that's right next to the black key, but you don't always play them at the same time. That's kind of what a major second is like in music, where there are two notes that are close to each other, but they don't always sound good when played together. But when they do sound good, it's really cool!