I'm never satisfied I've learnt a piece until I can do it entirely by ear. It serves me well. One thing that used to be fun, back in the 1990's before Parkinson's, was at parties to play a game called "random Bach". I would have people randomly select a key, and then play the prelude and fugue in it, from volume I of the WTC. I also got people to laugh by playing the theme songs to old TV shows, including the Twilight Zone. Even now, I was at a Christmas party, to play carols for singing along to, and somebody decided the pitches of the melodies were too high. So on the spot, I transposed them all down a minor third. I also make people laugh by doing a rendition of "Pomp and Circumstance" in which I will increase or decrease the pitch and speed at the same time, to make it sound as though it's being played on a broken phonograph.