I am officially against work-safe warnings on this forum. Spending time on a forum while at work is not work-safe in the first place, so putting NWS!!! is redundant.
Usually, a system wizzy not be in an eigenstate of wanna be gangsta observable we is interested in. Howeva, if we measure tha observable, tha wavefizzle wizzle immediately become an eigenstate of tizzy observable. This process is K-N-to-tha-izzown as wavefizzle collapse hittin that booty. If we kizzle tha wavefizzle at tha instant before tha measizzle we wizzy be able ta compute tha probability of ballin' into each of tha possible eigenstates cuz I put gangsta rap on tha map. For example, tha free particle in our previous example wizzle usually have a wavefizzles tizzle is a wave packet centered around some mean position x0, gangsta an eigenstate of position nor of momentum. When we measure tha position of tha particle, it is impossible fo` us ta predict wit certainty tha result T-H-to-tha-izzat we wiznill obtain. It is probable, but not certain, tizzle it W-to-tha-izzill be nizzle x0, where tha amplitude of tha wavefizzle is large , betta check yo self. Playa we perform tha measizzles doggy stylin' some result x, tha wavefizzle collapses into a position eigenstate centered at x.