In curved space, the angles of a giant triangle don't add up to 180 degrees. By measuring the apparent size of spots in the cosmic afterglow, the WMAP and then Planck satellites showed the observable Universe is flat, to better than one percent. Parallel light rays stay parallel. That still doesn't tell us whether the Universe is finite or infinite.
A large share of Earth's water molecules formed in the interstellar cloud that existed before the Sun was even born, more than 4.6 billion years ago. You are literally drinking a cosmic fossil.
By day, the Martian sky looks butterscotch because of fine dust hanging in the air. But at dusk, those same grains scatter blue light forward, creating a bluish glow around the Sun. Rovers have photographed these blue sunsets: the exact reverse of what we see on Earth.