A log will float in water because a log is less dense than water. If you take any number of objects/substances that don't mix immidiately (ex: water, molasses, oil, ice, a cork and/or a rock) and put them in a container together the items will settle into layers by density (salad dressing does this too). The bottom layer will be the most dense and the top will be the least. You can actually do this experiment yourself with a clear drinking glass as the container. Interesting factiod: Not all wood floats in water. There are a few types of wood (Ebony, Lignum Vitae) that are denser than water and therefore do not float.