@Article{Lee08,
  author    = {WeeSan Lee and Ruwanee de Silva and Eric J. Peterson and 
  	       Robert C. Calfee and Thomas F.~Stahovich},
  title     = {Newton's Pen: A pen-based tutoring system for statics},
  journal   = {Computers \& Graphics},
  pages     = {511-524},
  volume    = {32},
  issue     = {5},
  month     = {October},
  year      = {2008},
  issn      = {0097-8493},
  doi       = {http://dx.doi.org/10.1016/j.cag.2008.05.009},
  publisher = {Pergamon Press, Inc.},
  address   = {Elmsford, NY, USA},
}