Simple explanations

If it looks like a duck, and it quacks like a duck, then it's probably a duck.
It's funny that a concept so obvious has actually a name: the Occam's Razor, and it originates back in the 14th century. (Sometimes I think that in the Middle Ages it was relatively easy to make history!)
This principle states that the explanation for a given phenomenon that has the fewest assumptions should be preferred over more complicated ones and can be used for instance to try explaining the Fermi paradox on the fact that we haven't found any prove of intelligent extraterrestrial life even though many think it ought to exist.