1. A computer storage device storing computer-executable instructions that, when executed, perform acts comprising: displaying, on a device interface, an image and a set of candidate objects for object recognition, the set of candidate objects including a specific candidate object and one or more other candidate objects, the specific candidate object corresponding to a missing portion of the image, the one or more other candidate objects including at least a first candidate object that does not correspond to the image; detecting a selection of a candidate object from the set of candidate objects; and determining whether the selection was made by a human based on whether the candidate object corresponds to the specific candidate object.