our newest bad habbit

Life with a toddler is never boring. There are always new things to learn and do--some good and some bad. Just in the last few weeks, Campbell has learned to say "night,night," tell you what the cow, lion, and duck say, feed himself with a spoon, and walk pretty much where ever he wants to go. Those are all so much fun to watch and show off to other people. What a sense of pride you feel when your child learns something new, unless he learn this habbit.
Campbell's newest bad habbit is peeing on whomever is lucky enough to change his diaper. While changing a newborn's diaper, one expects to get peed on--or worse, but as the child gets older, those episodes are usually less frequent. This is what Campbell had done, until one day when Campbell, Jamie, Russ, and I were at the mall. I was changing his diaper at Parisians, and Jamie yelled, "Watch out, he's peeing!" We all jumped and laughed and thought it was funny. Well, apparently Campbell thought it was a little too funny because now he strains to do it each time you change his diaper. You can see his little tummy contracting and watch him hold his breath straining to get just a little bit out. And when he does, boy does he think it's funny! Some people don't believe me, until they witness it for themselves. My in-laws had Campbell Friday night while David and I celebrated our anniversary, and Campbell peed on my mother-in-law no less than 5 times! You tell me that's an accident.
And so, my question to you is the same question everyone poses to me when I tell them about this new problem--What can I do about this? I mean seriously, how does a 13 month old have that much control over his...faculties? I can only imagine what the people in the pediatrician's office would say if I called in to ask about this problem! Any ideas are welcome. At this point, I'll try anything.