mathematics, why does one plus one equal two?
mechanics, how do pistons work?
navigation, why did the plane change course many times while flying from L.A. to China?
nautical astronomy, How did the Polynesians in canoes get to the Hawaii’s Islands in 300-600 AD.?
heat, why is my home warm and cold according to my wishes?
Thinking is a terrible thing. If done correctly it ought to frighten you and shake you from your comfortable foundations, because it should reveal things, raise questions and challenge your ideas about yourself, your beliefs and the world.