The external tank for the shuttle contained tanks of liquid Oxygen and Hydrogen for the main engines to use. These two have similar (though not equal) volumes, but the Oxygen is heavier in an obvious stochastic sense.
Given that, I just assumed that the heavier fuel (Oxygen) tank would be on the bottom and the lighter one would be on top, to make the center of mass lower. It seems like a lower center of mass would make it less likely to tip over during the climb. But that's not how NASA did it. The Oxygen tank was on top. Why?