"Why" do ice creams have to melt??

"Why?" is the thing of the moment, for my best friends 4 year old. The answer by the end of their 3.5 hour visit was "Because it just is/does!" Explaining everything to within an inch of its life does get very tiring. I'm sure that I don't have to think that much whilst I'm at work. Fabulous fun and sun filled day off x