This afternoon. First, we tried a science experiment (about carbon dioxide being heavier than air, and fire needing oxygen to burn) which we thought would be a good visual way of getting the point across, which didn't work, but was quite funny as we attempted it!
My sister (aged 15) then asked me to teach her something on the piano. She meant something vaguely "pop"-like, but I got out a book of easy arrangements of classical well-known stuff, and she then spent ages trying to play the first bit of one piece. Her concentration made my day!
We also did some singing stuff (as at first she seemed unable to pitch a note after I played it on the piano, but could if I sang it - slightly concerned me! She managed it eventually!) and she grabbed a hymn book from on top of the piano, and sang various hymns she found flicking through!
She never usually takes interest in piano or music apart from her "pop"-stuff which she sings around the house, but today she did.