I'm a type of problem that makes your brain work hard, often called a tough 'nut' to crack. What am I?

Which small deer's name suggests it holds the answer? Solve the riddle → The sun sends out particles that dance in our atmosphere, painting the sky with magical glows, far from warmth. What am I? Solve the riddle → I am always wet, even though I die every time I touch the ground. Solve the riddle → I am your window to the world, but also the world's window into you. Solve the riddle → The more you postpone me, the heavier I become. What am I? Solve the riddle →