Ever heard of a gas called Helium? It’s used in blimps and kiddy balloons. It’s the gas that makes a person sound like a munchkin when it’s inhaled (assuming they didn’t sound like a munchkin before inhaling).
Young Earth Creationists think the amount of Helium in the atmosphere is too small (no, not because Creationists want everyone to sound like munchkins).
They claim that at the rate Helium is produced by radioactive decay, there should be several times the amount of Helium as currently exists if the earth is billions of years old.
This however ignores the fact that Helium is a very very light gas. So light in fact that it escapes into space. The reason why there is less Helium in the atmosphere is because it eventually escapes into space after it’s produced.
Some Creationists have adapted their argument to claim that the rate of escape vs. production by radioactive decay is still wrong, but it’s their figures that are wrong.