How many objects are inside this box?

To judge the contents of a box, we do not necessarily have to open it. By shaking a box we can make an estimate of its contents based on haptic and auditory information. Not much is known about the perception of properties of objects that are inside a box. In this study we investigated how accurately participants can judge the number of wooden spheres inside a small handheld box by shaking the box. This was done in a `haptic + auditory' condition in which participants shook the box and in a subsequent `auditory only' condition in which recorded sounds from the trials in the haptic + auditory condition were played back. In both conditions participants had to judge the number of spheres (1 to 5) inside the box. In the haptic + auditory condition participants could perform this task accurately for up to about 3 spheres, while for larger numbers they systematically underestimated the numerosity. Although participants could perform this task above chance in both conditions, accuracy was lower in the auditory condition than in the haptic + auditory condition. By actively shaking the box the number of objects inside can be judged accurately for up to 3 objects.