The next time someone asks you, don't tell them it's simply because gas molecules in the air absorb light waves with short wavelengths and then reflect them back out (or, God forbid, because "the Lord made it that way"). Instead, draw on this wonderful piece from the National Post and answer that it's not blue -- it's violet:
The violet wavelengths from the sun, having still shorter wavelengths than blue, should be scattered even more. Given this, shouldn't the sky be violet, not blue? Indeed the sky is... READ ON