|When magnetic tape is put on top of another piece of magnetic tape, the tape on the bottom is demagnetized — its memory is erased. How are we then able to rewind and forward tape, scrolling the tape together, on a cassette tape with out damaging the magnetism?
The process of winding tape up on reels does damage its magnetism slightly. The adjacent layers of tape do interact with one another and they do cause the sound on one layer to appear on the adjacent layers. Fortunately, the effect is very subtle and takes a long time to appear so that the tape must remain tightly wound up for ages before you can hear the damage. Tapes don't age perfectly anyway because thermal energy slowly erases the magnetization, particularly in a hot environment.