Geography
Demographic Momentum and Its Implications for Population Growth Projections
Quick fact
China's population would have continued growing for decades even after its one-child policy reduced fertility below replacement level, because the huge cohort born before the policy would still have children—a direct consequence of demographic momentum.
Why this is interesting
Imagine filling a bathtub with the tap on low: even after you turn it down to just a trickle, the water keeps rising for a long time. How can a population keep growing even after every family starts having only two children?