Simple question if it's already been asked a mod can lock but I want to know if Clark wanted could he be stabbed by a normal knife? No kryptonite or any type of other trickery just a normal knife. Could he allow it to pierce his skin and make a wound?

No he doesn't have that type of control.

Sounds really wrong to me. You can't do that, and his phisical prowess is just like yours.

I thought that'd be the case.

Nope, it's a passive thing. He can't change his skins permeability any more than you can.