Trust issues

The 4 y/o is having a drink of water from a blue beaker, and there's a half-full glass of water on the table from one of the other children.

4 y/o: I don't trust this clear glass. I only trust this one.
Me (unaware until now that my child had trust issues with any crockery or household items): Why?
4 y/o: Because fruit flies.
Me: ???
4 y/o (matter of factly): Fruit flies swim in water to get fruit. If you put a pear in a yoghurt and left it out, fruit flies would kind of swim to get it.