What is Up with Zhu Zhu Pets?

I have yet to hear my girls shout they want Zhu Zhu pets, but I keep seeing them everywhere. I went to Wal-mart a few days before Black Friday and saw an entire rack empty and on it said, "Zhu Zhu pet toys: One per customer" and asked the hubby who had no clue what they were. Later I found out these were a HOT item with little girls and even tween girls.SoccerBoy informed me that girls in middle