A quick math adventure: Estimate the number of co-authors in this publication (“Erratum to ‘Atmospheric effects on extensive air showers observed with the surface detector of the Pierre Auger observatory‘”, Astroparticle Physics, vol. 32, no. 2, 2009, pp. 89–99. Thanks to investigator Adrian Smith for bringing it to our attention.) If you want to do it the hard way, and actually count, click on the image.
By the way, Aaron Santos’s estimable new book How Many Licks? is all about how to estimate.