Why are false memories sometimes entirely convincing

Existing knowledge and other memories can interfere with the formation of a new memory, causing recollection of an event to be mistaken or entirely false. Imagine if you had two puzzles and you mixed them both together. You're trying to build puzzle number 1, but the pieces of puzzle number 2 also fit into puzzle number 1. So you accidentally build puzzle number 1 successfully but with the pieces of puzzle number 2 in it.