Some time ago I bought two nearly identical pairs of shoes; one black, one brown. They're the same "style," manufacturer, texture and everything; they're just different colors.

It happens that when I wear my brown shoes, though, I stumble a little. It's like the sole of the shoe catches on the floor. I'm not snagging my toe or catching the heel, like one might expect when stumbles. It's more like the flat part of the shoe, under the wide part of my foot, hangs a little lower on the brown ones, and as my gait swings forward, the bottom scuffs on the floor.

I've noticed this for some time. I have started paying special attention to my gait as I walk in both pairs of shoes, and I can't tell anything different. The shoe itself isn't sagging at any point. The brown ones don't weigh more than the black ones. They're the same size, both in shoe size and external dimension.

But when I walk down the hall in my brown shoes, I stumble.

Weird, huh?