How to hear the difference between .mp3 and .wav?

.wav files are uncompressed data, are they not? That is what an audio CD is made of I believe. MP3 files take adavantage of similarities in the data and compress it to use much less space yet sound very similar to the original. If the compression is great, the file becomes much smaller, but the sound will become noticeably tinnier and worse. An MP3 file of a song encoded in 64 kbs should be slightly (but noticably) worse than the same song played from the original CD. Try it.