Joseph Farrelly of Nelson Creek School asks :-

What goes up but never comes down?

John Campbell, a physicist at the University of Canterbury, responded.

The usual answer is taxes and wages, but very occasionally a serious depression makes a lie of that.

Can you think of anything in Nature where this happens? For example, bubbles emitted by a diver underwater. Except that in turbulent water they can go down for part of the time they exist, but overall they rise to the surface, or the roof of a cave. A helium-filled rubber-balloon is lighter than air so it goes up. Except that after a day or so enough helium gas has diffused through the thin rubber that the balloon becomes heavier than air and descends.

Hydrogen gas is lighter than air so it goes up. In fact it keeps going up. At Earth temperatures the light molecule has sufficient kinetic energy that the speed of the molecule between collisions with other molecules is fast enough that it exceeds the escape velocity of the Earth. Why? If you throw a ball upwards, the gravitational attraction to the Earth causes it to slow down and then return to the Earth's surface. Imaging firing it upwards faster and faster. Eventually there will be a speed, the escape speed, at which the ball doesn't return. That happens when the kinetic energy of the ball just exceeds the gravitational potential energy at the Earth's surface. So hydrogen gas at the top of the atmosphere has sufficient speed to just keep going away from the Earth. That is why there is no hydrogen in the Earth's atmosphere.

If you consider "up" to include the meaning "increase", the amount of lead on Earth goes up. Lead is the end-product of the disintigration of many long-lived radioactive atoms, such as uranium and thorium, which occur naturally in rocks and soil on Earth, so is continually being manfactured in Nature. Likewise, Helium (old alpha particles) is another by-product of radioa-active decay. But, like hydrogen, it can escape from Earth so the total amount of helium in the atmosphere is small, whereby the production rate through radioactive decay equals the loss rate. Once hydrogen or helium atoms leave the Earth the concept of up is meaningless.

I think I am right in saying the only thing that keeps going "up" is entropy, the scientific measure of the disorder of systems, including the universe.