In theory, the transition between the two principal superfluid forms of liquid 3He ought not to be able to occur. So why is it routinely observed to take place? A possible answer to this intriguing question has recently been put forward by A.J. Leggett. Writing in Physical Review Letters 53, 1096; 1984, he suggests that the transition, although prohibited from occuring spontaneously, readily takes place because of the occasional high energy cosmic ray that passes through any experimental chamber on the Earth's surface.