This was taken while chopping up some wood from a few recently cut down trees. The tree had been dead for quite a while, and so though the wood is still considered green and won't be good to burn until next year, the wood is dry enough to split fairly easily. As you can see, I was able to open a good crack on the first swing, and it's went through on the second.

This has been submitted to the "Tension" contest. It's a bit literal, but log splitting with an ax is an example in several principles of physics. What is keeping the blade stuck in the wood is tension applied on both sides of the metal. What makes the ax work in the first place is momentum, combined with a design that converts that force into a relatively small amount of surface area (the sharpened edge).

It's been a while, but I did like my high school physics class.

Taken with my Nikon D5000: f/4.5, 1/250 sec, iso-320