Greetings, friends. I'm new here, but if I may, I'd like to repeat but slightly rephrase the answer that Morte, PaulK, contracycle, and Maxwell's Demon have suggested.
The key difference between a crop circle and a living cell (or other biotic object) is that the former corresponds to artifacts which we know from experience to be intelligently created. The latter does not. In other words, we base our evaluations of these things on whether or not they are precedented in our mental catalog of events in one category or the other.
To be sure, this is not a foolproof methodology. We may be entirely incorrect in assuming that a cell is not intelligently designed. But we're certainly justified in making that assumption with the information available.
Siempre que enseñes, enseña a la vez a dudar de lo que enseñas.