 So earlier today, we talked about the fact that calcium comes in, binds to troponin, and moves tropomycin out of the way, so that actin and myosin can now reach each other and perform the sliding filament theory. But everyone always asks, why calcium? Why not something like sodium? Or even chloride? Why does it have to be calcium? And the reason why is that if you look at ions, okay, if all of these keys are ions, then they all have similar properties. But if I was to just take something like sodium and fit it into my troponin, it doesn't work. And it's because it doesn't have the right size or shape or property in order to bind into this hole that's available. The only one that will fit is going to be my calcium. When I insert my calcium, okay, and it binds to troponin, there's going to be a structural change that happens where it actually moves, okay, and opens up the interaction for actin and myosin to happen by moving tropomycin out of the way.