Astronomy

Why does the same side of the moon always face the earth?
Why is the night sky black?

Dynamics

How can a sailboat tack into the wind?
Which way does a reversed (i.e. sucking) rotary lawn sprinkler turn, if at all?
Why does a spinning top not fall over?

Optics

Why does a mirror reflect left/right (right-to-left) but not up-down (upside-down)?

Terrestrial

Why is the sky blue?
Why is the evening sky red?

Thermal

Why do whites keep you cool in the summer sun?
When should I pour the cream to keep my tea or coffee hottest?