You have to warm it slower and make sure your water is no more than about 130F that you're using to warm it. There are two ways to cause mouseplosions: the snake constricting exceptionally hard (probably not the case), or cooking the rodent. I had it happen to me when I was using boiled water to warm em up, but not since I switched to hot tap water (change it once when it gets cool and give it another 15-20 min and it should be good).