Intuitively, though, we tend to think we'll always know design when we see it. For instance, the ridges on this stone pottery found in British Columbia clearly indicates that it was spun on a lathe, and holes have been drilled through the center:
Clearly, this pottery must have come from an ancient civilization... except it's not pottery, and these objects are naturally-occurring. They're carbonate concretions formed through a complex geological process, although objects like these really were misinterpreted as artifacts by pseudoscientific writer Graham Hancock.
Okay, so we're zero-for-one. Next is a charming, picturesque ring of mushrooms planted as decoration in a city park...
...by which I mean, a fairy ring created as a natural result of an interconnected underground network of mycelia. Cultures of the past came up with many fanciful explanations for this phenomenon—generally involving the intervention of fairies, elves or some other intelligent agent.
All right, let's try this again. What about these cement pylons? This group of carefully shaped hexagonal bricks could serve not only as a protection from the erosion produced by incoming waves, but also as a staircase up to the shore.
Nope. These are basalt columns in the Giant's Causeway, caused by the rapid cooling of thick lava flows. Curses, foiled again.
Okay, here's the last one. This has got to be intelligently made, right? I mean, just look at those clean right angles. These are obviously the remnants of an ancient building, or some type of crop irrigation system:
...Or maybe it's tessellated pavement on the coast of Tasmania, created by a rare combination of stress cracks and erosion facilitated by the accumulation of salt crystals. Dammit, this design detection business is harder than it looks.
Of course the creationi—er, intelligent design supporters—may retort that these phenomena are simple compared to the incredible complexity of biological systems. But what I've provided here is just a proof of concept, showing that phenomena that appear to be thoughtfully designed for a specific purpose can undeniably be the result of natural forces. The crucial thing to remember is that life has a complexity-building mechanism that blows the ones displayed here out of the water: natural selection.