Grade: B+

Most video games challenge the player’s problem-solving skills, reaction time or hand-eye co-ordination. But a handful of them offer satisfactions of a different sort: the gentlest of difficulty curves and the calming pleasure, instead, of a mildly absorbing repetitive task which whiles away the idle hour in the way you might pass it flicking through a set of worry beads or making a cat’s cradle with a ball of string.