  Contextuality is a fundamental property of quantum theory and a critical
resource for quantum computation. Here, we experimentally observe the arguably
cleanest form of contextuality in quantum theory [A. Cabello \emph{et al.},
Phys. Rev. Lett. \textbf{111}, 180404 (2013)] by implementing a novel method
for performing two sequential measurements on heralded photons. This method
opens the door to a variety of fundamental experiments and applications.
