I highly suggest that people check out/try out "ABX comparator" plugin for Foobar2000 player. You give it two files (say a 128kbps mp3 version and a lossless flac verison) and it runs you through a double blind test of the two versions, quizzing you along the way. It's not just "listen to A" "listen to B", it gives you lots of options to listen to them and compare them in different ways, including obfuscating what track is A and what track is B.

If you think you can really hear fine differences in audio, it can be very eye opening (ear opening?).