Why is an empty shampoo bottle so easy to knock over?

It becomes annoyingly easy to knock over a shampoo bottle when it's nearly empty. This is an easily observed and annoying phenomenon that, according to Lehigh University physics professor Jerome Licini, yields insights into center-of-mass and impacts. Licini, along with first-year physics major Allen Zijun Yuan, wrote a paper on the phenomenon that was recently published in The Physics Teacher.