[SirReal]

I know the semester is over, but I still have a nagging question from some of the material we covered.  I am familiar with the mechanism of HBr addition to alkenes through formation of a carbocation, but how does HBr add to ethene without producing a primary carbocation? I saw a few diagrams in which hydrogen forms a cyclic bond with the two alkene carbons, much like oxygen in epoxides, but this baffles me.  I've never seen a hydrogen atom act that way.  Is there some peculiar mechanism behind the addition of HBr to a molecule containing only 2 carbons?