It's called a Vigenère cipher. I don't know how it works and I'm not too concerned about learning, what's important is that each coded message needs a keyword to unlock. I've figured out the keywords for the four messages I've received so far and they all decode to make riddles;
"What has existed for a million years but is never more than a month old?"
"What goes up but never comes down?"
"It cannot be seen, it weighs nothing, but when put into a barrel, it makes it lighter. What is it?"
"Until I am measured, I am not known, yet how you miss me when I've flown."
Okay, I figured out the code, can I get a hand with the riddles? I've never been very good at them.