umbrellas

Lillian and Nathan found our umbrella. Needless to say, it keeps them entertained for a while. It is really cute to hear them playing because they don't say it's an umbrella. They call it a rainbrella. Now, I think this new word makes more sense than umbrella. What is an "um" anyway? I'm not sure if I know the reason for calling it an umbrella and if I try to explain it to a 3 year old I'll probably convince myself that it should be called a rainbrella.