ABSTRACT
Some things happen, some things don’t: (i) Niagara Falls falls down, but not up. (ii) When left on the kitchen table, a hot cup of coffee cools to room temperature, but a glass of water at room temperature doesn’t heat up to 80°C. (iii) A gas under pressure expands to fill a vacuum, but a gas doesn’t spontaneously compress and move to one side of the container. (iv) Salt and water mix to form a salt solution, but a salt solution doesn’t separate into salt and pure water.
