A half-life analogy
If you had 1000 quarters and threw them up in the air you would expect about 500 to land heads up and about 500 to land tails up. (Of course it might not be exactly 500 and 500, but it would be close.) Now imagine that your brother took all of the quarters that landed tails up. Now you have 500 quarters. You throw them in the air and, again, about half of them will come down heads up. If you and your brother continue this process (you throwing up the quarters and he stealing the “tails”) you will have 250, then 125, then 62 or 63, etc.
Now, if we assume that each flip is a half life, that heads are still reactants and “tails” are products that go away, then this acts just like a first order reaction. With each flip you end up with half of what you had before, no matter how many that was.