A 2005 study led by Margo Lillie, a zoologist at the University of British Columbia, and her student Tracy Boechler, concluded that tipping a cow would require a force of nearly and is therefore impossible to accomplish by a single person. Her calculations found that it would require more than four people to apply enough force to push over a cow, based on an estimate that a single person could exert of force. However, since a cow can brace itself, Lillie and Boechler suggested that five or six people would, most likely, be needed. Further, cattle are well aware of their surroundings and are very difficult to surprise, due to excellent senses of both smell and hearing. Lillie and Boechler's analysis found that if a cow did not move, the principles of static physics suggest that two people might be able to tip a cow if its centre of mass were pushed over its hooves before the cow could react. However, cows are not rigid or unresponsive, and the faster humans have to move, the less force they can exert. Thus Lillie and Boechler concluded that it is unlikely that cows can actually be tipped over in this way. Lillie stated, "It just makes the physics of it all, in my opinion, impossible."