Embodiments of the present invention relate to an apparatus (i.e. an automatic teller machine) comprising an optical sensor. The optical sensor is configured to detect thickness of paper. Advantages of some embodiments of the present invention are that by determining a thickness of paper, it can be confirmed that two pieces of paper are not stuck together. For example, if two substantially identical pieces of paper are stuck together, then their thickness will be approximately twice the thickness of a single sheet. Accordingly, upon detection of two or more sheets of paper stuck together, a device, that automatically handles paper, may cause the sheets of paper to be separated.