I use an ex-pen like you recommend in your training DVDs. The problem is when I leave the room my puppy climbs up the side and falls out. At what age should I start to try and correct this behavior?

I picked up my 8 week old puppy today, I ordered one of your ExPens in advance. I have a slight problem. After exercising my pup for quite a long time today after her long plane ride, I placed her in the pen. I went into the kitchen for a moment and as soon as I returned into the den, she had already climbed up the side and basically fell on the floor. She wasn't injured at all and seemed to enjoy it. My question is at this age what should I be doing to stop this? It amazes me that she was even capable of performing this feat at such a young age. Have you ever had this problem? And at what age do you think that this will stop? Do you have any advice? How often do your dogs stay in the ExPen compared to the crate at the age of 8 to 12 weeks of age?