... that Lois only hurts her hand when she hits Clark after she's found out the secret?

That does seem to be true. Fanfic writers have a theory for that... that Clark sort of softens his body or makes himself less dense when he knows someone's going to hit him so that the blow is absorbed a bit.

Though... there was one time in the series... not sure what episode... where Jimmy runs into Clark while Clark's not paying attention and it really hurts Jimmy's shoulder.

I'd like to think that it hurts Lois *after* she knows because Clark's being a brat.

Sara

The Jimmy incident was either in Forget Me Not or Oedipus Wrecks

You'd think that would get Jimmy thinking, maybe jog his memory of all of Clark's lame excuses. I wouldn't be surprised if he figured it out before Lois did, but just never said anything.

Especially after that VR ep (or am I thinking of Forever Knight?) Both shows had VR eps IIRC, and both had asteroid-related eps. Interesting coincidence.

No, there was an episide in LnC too - Virtually Destroyed.

Infact, there's a fanfic wherein Jimmy finds out after seeing Clark glass-less in virtual world. I forget the name tho...

The Jimmy running into Clark incident was in Forget Me Not. Though I remember Lois commenting that Clark had shoulders that felt like iron when she massaged him after his apartment was broken into I think it was The Foundling but I can't remember I wasn't that big a fan of L&C back in the first season.