1. The problem statement, all variables and given/known data Why are mammals as large as they are and not much larger? 2. Relevant equations Two figures are given. One of the mammal with size S and mass M. Femur is the thigh bone of the mammal with l being the length of the femur, d being the thickness of the femur and A being the cross sectional area: i)l ∞ s - length of the femur is proportional to the size ii) m∞s3∞l3 iii) pressure ∞ weight/Area ∞ m/d2 iv) m ∞d2 v) d2∞l3 Therefore, d ∞ l 3/2 3. The attempt at a solution These have been worked out, but I am confused at how length is proportional to the size. Is volume the size? In fact, I don't understand the whole thing. Please help. I have looked at the MIT video three times and still confused. Sorry for being so dumb but I don't hesitate to ask.