The shape of the tree is its "skeleton", with the trunk and branches making up its bones. Some of those bones are white, some are dark. Some are smooth, some rough.
Grab your sketchbook or journal and go on a tree skeleton hike. Draw the structures of your neighborhood trees - and maybe do a rubbing of the bark. It's a good way to get to know the trees without their leaves on.
Today is STEM Friday - head over to the STEM Friday blog to see what other people are talking about in science, technology, engineering and math.