Unlikely Analogy of the Day: Nucleophilic Substitution and Girlfriends
The reaction needs a substrate and a nucleophile: the nucleophile bonds to the substrate replacing the part called leaving group - hence the substitution. It proceeds through two distinct mechanisms: SN1 and SN2. SN1 entails initially the loss of the leaving group with formation of a positive ion from the substrate (carbocation), which then reacts with the nucleophile to give the product(s). SN2 instead is a concerted mechanism: the nucleophile attacks the substrate and pushes the leaving group away yielding the product. Certain substrates manifest ambiguous behaviour, of course.
How does this relate to girlfriends, tho? This way...
A relationship ends for some reason, and the guy remains single, ready to attach himself to another girl. This is analogous (with some fantasy) to the SN1 mechanism.
In the other case, a second girl gets close to the couple, bonds to the man and pushes the former girlfriend out, in a fashion resembling the SN2 mechanism.
Threesomes are not stable, much like the transition state for SN2 with its pentavalent carbon atom isn't.
And now I want my engineer's disease certificate, pronto!