[He quirks a brow at that statement. "Essentially" seventeen? Sure, Lucifer is "essentially" something like forty, but that's because Lucifer is a demon, and demons sort of just do that living forever thing. To him, Senku has the presence of a normal human, so that's... hmm. Interesting.]

I'm interested to hear more about that. It's extremely hard to surprise me. You could say I've seen a lot in my time.

[He's not directly asking for information, but the intent behind his statement is likely obvious to anyone with a brain. Lucifer figures they can talk between chords if necessary, so he continues on, explaining to Senku briefly the names of the white keys first as a starting point and then adding,]

Put your left hand here, and position your fingers each with a key between them. The keys under your ring, middle, and index finger make up the chord of F major — the F, A, and middle C keys. If you were to move your hand approximately one key to the right, still leaving space between your fingertips, that's the G major chord.

[He's a patient teacher, at least, taking his time to explain the common chords and notes in detail. The black keys are for later chords.]