Majority of chihuahuas have a soft spot on the top of their head called a fontanel or molera. Back in the day the open fontanel was a sign of purity in the chihuahua breed. It is a common breed trait and does not mean that dog has "water on the brain". Typically the more domed a dogs head is, the bigger the fontanel. You can find information on open fontanels on
Chihuahua Club of Americas site.
