Know by heart

Meaning:

If we know something by heart, we know it from memory. We do not have to read it to remember it. We do not need anybody to help us remember it. We remember it automatically. This idiom can be very useful when we're talking about phone numbers or other bits of information that we have in our heads. It can also be useful with songs or poems.

Example sentences:

1. I know it by heart.
2. Do you know that song by heart?
3. You need to know this by heart.

Example dialogue:

Two friends are talking about their favourite songs.

A: Do you know any songs by heart?
B: Sure. I know many Beatles' songs by heart.
A: Cool. I know some U2 songs by heart.
B: Oh, I like U2, but I don't know any of their songs by heart.

Practice:

What songs do you know by heart?