It’s a common question, and it isn’t easy to explain, since much of it is complicated while the simple parts are often counter-intuitive. But here’s a
comic that tries and illustrates the problem.
Here’s the correct explanation, that actually jibes with the evidence.
Here’s the ID/creationist explanation:
Then, of course, in the competition of ideas, the two hash it out and…well, you’ll have to read the whole thing yourself. Sad to say, the ending rings true, too.