If you were to ask me how do I know the sky is blue?
I say through my visual sense I observe the heavens looking up to witness it. If I tell you "oh I use critical thinking skills", that's not telling anything.
That's why you haven't elaborated passed that in the practical sense I just gave.
I'm always on about this. We teach the kids "The sky is blue, the grass is green, cows say moo cats say meow."
None of this is very accurate so we teach the kids to believe certain things but not to think very well. Everyone knows the sky is blue so there it is.
The sky appears blue to us sometimes. Sometimes it appears grey, white, red, purple or a sort of mauve, pink, orange, yellow, black, sometimes black with white spots, and I've even seen it green. Grass is most often green, but sometimes yellow, brown, black, bluish and so on.
It sounds silly, and in a way it is, but damnit! [pounds fist on desk] something needs to be done about these transgressions!
ETA: Oh, I forgot. Cows and cats don't make an m sound.