"That's not true."
Almost every day my six-year-old comes home from Kindergarten and shares a little tidbit of wisdom she got from a teacher. It's usually something profound like "green means go, red means stop" or "sugar causes cavities" (which didn't seem to impress her when I told her that).
The other day she told me, "Water is healthy; tea is not."
What could I say? "That's not true." (With caveats like "if that's what he really said.")
I was briefly concerned about undermining her teachers, and I certainly don't want her going to school and challenging one, but I decided she should learn early that everyone is fallable, even her teachers. (She already knows that about me.) And given the state of public schools, it can't hurt to start her off early with the knowledge that a teacher that contradicts Mom and Dad can be wrong.