Music Album

I just remember about this. My dad told me about this before but I just noticed it again. Do you guys notice that in the music CD, usually the single or the famous song is placed on track 1 or track 3. I just checked my albums and most of the singers put their first single on track 1 or track 3. Ok, it makes sense to put it on track 1 because listeners would want to hear it right away when they play the CD or cassette. But why no 3 though... hmmm...