I'm having trouble understanding why both methyl groups on top of the ring is a more stable conformation than both at the bottom of the ring.
I found a reasoning that says the steric strain between two -CH3 groups are less when both groups are on top. (Figure (a))
Are the methyl groups closer to each other when they are both under the ring? Can this have something to do with the ring structure being a little 'twisted' or 'puckered' to minimize torsional strain?