A strain-responsive visual indicator comprises a pair of overlapping shutter strips (1, 2) adapted to be mounted to a substrate such that strain in the substrate causes relative movement M between them. Each shutter strip comprises an alternating set of windows (5) or (6) and bars (7) or (8) and the distal strip (2) is backed by a strip (3) of a different color exposed through its windows (6), or the distal strip can instead be printed with a set of alternating colors in place of its windows and bars. The effect is that under different strain conditions different color indications from the distal strip (2) will be visible through the windows (5) of the proximal strip (1), in accordance with the relative positions of the two strips. The indicator can be incorporated e.g. in the shaft of a toothbrush to give a visual warning in response to flexure of the shaft if excessive brushing pressure is applied. Numerous other applications and the use of different types of indicia are indicated in the specification.