You ought to 'get cracking' [='get going; get moving'] on that assignment. [=you should start working on that assignment right away]
The two of them 'crack' jokes all the time. = The two of them are always 'cracking' jokes.
The stress was getting worse, and she felt herself 'cracking up'. [=going crazy]
(..) - Sometimes I can't hear anything. - Is that so? Here's my card. If you recaII anything unusuaI about Iast night... anything at aII, caII me. You bet. Thank you. And good Iuck... cracking the case. Oh, my God, I'm Iate! Keys, keys... Where's the keys? Damn. Get the keys. (..)1994 The Mask