Newton's laws of motion: Isaac Newton discovered three laws of motion.  Like all scientific laws they describe the observed behavior of matter.

• First Law: every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.  This is also known as the law of inertia
• Second Law: the force on an object is equal to its mass times its acceleration.  F = ma  The unit of force is the Newton and is equal to the force required to change the velocity of  a 1 kg mass by 1 metre per second in one second (1 kg·m/s² = 1 N).
• Third Law: for every action there is an equal and opposite reaction

Nitrogen tetroxide: properly called dinitrogen tetroxide, N₂O₄ is a colorless toxic gas at room temperature.  Since it is a dimer of brown NO₂ it will usually not be clear, but have a slightly brownish color.  It is a common oxidizer for use in spacecraft, since its boiling point (21 ºC) allows it to be stored as a liquid under pressure without refrigeration.