A CRC is the remainder or residue after division (by the CRC generating polynomial). So you are given a dividend (the message) and a remainder (the CRC). What

A CRC is the remainder or residue after division (by the CRC generating polynomial). So you are given a dividend (the message) and a remainder (the CRC). What is the divisor (the CRC polynomial)? From one example dividend/remainder you can come up with several possible divisors, but you have already greatly narrowed the field. With more examples you can eliminate the false positives.