Talk about unlikely heroes. The staid old Blackberry has accidentally acquired a status for which many of its competitors would lop off a limb: it is a teenage phenomenon. The reason behind this strange success is instructive, though hard for competitors to replicate. The Blackberry’s web-browsing is poor compared with peers; ditto its music offering. But teenagers and students have adopted it for its instant messenger system, which allows SMS-addicts to have text conversations in real time for free. Sarah O'Connor analyses the trend.