Hacks at the Massachusetts Institute of Technology are practical jokes and pranks meant to prominently demonstrate technical aptitude and cleverness, or to commemorate popular culture and historical topics. The pranks are anonymously installed at night by hackers, usually, but not exclusively, undergraduate students. The actions of hackers are governed by an extensive and informal body of precedent, tradition, and ethics. Hacks can occur anywhere across campus, or occasionally off campus; many make use of the iconic Great Dome, Little Dome, Green Building tower, or other prominent architectural features of the MIT campus. Well-known hacker alumni include Nobel Laureates Richard P. Feynman and George F. Smoot. In October 2009, US President Barack Obama made a humorous reference to the MIT hacking tradition during an on-campus speech about clean energy.