A parable. I have just started school, and I find out that many of my classmates do not believe in Santa Claus, or at least not in most of the stories that have grown around him. They say that the toys we get on Christmas were not made especially for us personally by magic elves, but were ordinary store-bought toys purchased by our parents. I don't believe them, but being a bright and curious fellow, I decide to investigate. I'll prove that Santa is real. I find some toys hidden in a cabinet in the garage. They are still in the bags from the stores in the Mall, and they all have reciepts from those stores (the fossil record). I also find my parent's checkbook with the record of purchases in those same stores on the same days as the reciepts and for the same amounts (the geological record).
I could come up with a complicated explanation that preserves both the purchase of the toys by my parents, or I could fail to notice a key point of evidence, even when someone keeps trying to get my attention with it, or I could claim to have considered the evidence but found it unconvincing. After all, no one actually saw my parents buy the toys, so we don't know that they did, do we?
Or I could follow the basic principle of science that the least number of unfounded assumptions makes for a better working hypothesis. One assumption, that my parents purchased the toys to be given to me "from Santa" on Christmas morning fits all of the evidence.