Spring time can be a wet time. So when you're biking, nothing is more annoying than having mud splash up onto your butt and back (especially if you're biking to work).
You could pack a change of clothes, but why not stop the problem where it starts? Since you can't stop the rain from falling, you've got to stop the mud from splashing. Do your clothes a favor and get a mudguard.
This one is called the Flinger Portable Rear Mudguard. What's so great about it? For one thing, it's cheap ($12.99), and can be easily attached and detached. Once you take it off, it can retract (like a pirate's telescope) to a compact size, and placed in the convenient little storage bag that comes with it. So after you get to where you're going, you can stick it in your backpack.
Another great thing is that it's got reflective trim on it. You really can't have enough of that stuff when you're a biker.