Oxygen-18 is a heavier-than-normal isotope of oxygen.
Statements
Oxy-18 is less common than normal oxygen but heavier.
In a rain cloud, most of the water has normal oxygen, but when the cloud rains, a higher percent of the oxy-18 falls out.
Paradox
Since the oxy-18 drains from a rain cloud faster than the normal oxygen does, scientists were confused when they tracked a bunch of clouds and saw that the oxy-18 content was steady, even though the clouds were definitely raining a bunch.
Evaluate
In case a non-math-y metaphor would help, picture a giant bowl of Skittles. A minority of those Skittles (20%) are red ones, but red ones are more delicious than non-red ones.
Thus, red ones leave the bowl at a faster rate than non-red ones do (as people walk by, they're more likely to grab red ones out).
You'd expect that after a lot of eating, there would be very few red ones left (like only 5% red ones). But instead, someone came by the bowl and saw that there was still 20% red. How could that be?
It suggests that someone is putting red ones back into the bowl at a higher rate than the other colors. That's how, even though reds are leaving at a faster rate, we could preserve the same ratio of red to non-red.
Goal
The paradox tension suggests that clouds are somehow getting oxy-18 put back into the clouds as a faster rate than normal oxy is put back into the clouds.
But we should stay flexible and pick whatever answer we think is most provable. Beware strong language, unknown comparisons, and new ideas.