One of the common overlooked first lessons of biology is determining if life exists. There are seven characteristics/factrors that must be met in order for an object to be considered living.
The attached worksheet is a solid homework/in-class activity that focuses students attention to the basic characteristics of life. The worksheets present various conditions/characteristics and students must pick out the seven correct choices to unscramble the "code" and solve the worksheet. This works well as re-enforcement of a lesson and also as timed activity that allows students to "compete" to solve the code.