diff --git a/photon.md b/photon.md
index 76af421cf..c59564588 100644
--- a/photon.md
+++ b/photon.md
@@ -2,3 +2,4 @@
   - "[[Einstein]] was led to the idea of light quanta which has proved so fertile"
   - "…two photons, as seen by a single observer, differ in direction of motion, in energy and in polarisation".
   - "If we were moving with rapid acceleration toward a wireless station, its photons would appear to possess increasing amounts of energy, and would pass over the whole spectral scale through the visible and into the ultraviolet"
+  - "Even so fundamental a process as the flow of heat must involve two factors, the amount of energy and the number of photons transferred"