Our elbow bends all the time, we bend it when we pick up things, when we eat, when we do mostly everything. Cows on the other hand dont pick up food with there feet, they use there mouths, and they probably only bend when they are walking.
Perhaps the answer is "physical characteristics". I am simply making an educated guess, however I do know early philosophers classified organisms into two groups, Plants and Animals, and identified them based on physical characteristics.
1. theyre opposite because during photosynthesis CO2 is absorbed by plants and during respiration CO2 is emitted. 2. glucose is made during photosynthesis and serves as food for every (!) animal, and animals decompose glucose to CO2 during cellular respiration.
