diff --git a/300s School/NUCE 2100 - Fundamentals of Nuclear Engineering/2024-09-17 Module 5 Nuclear Fission Basics.md b/300s School/NUCE 2100 - Fundamentals of Nuclear Engineering/2024-09-17 Module 5 Nuclear Fission Basics.md
index b27154a8..9e7c2215 100644
--- a/300s School/NUCE 2100 - Fundamentals of Nuclear Engineering/2024-09-17 Module 5 Nuclear Fission Basics.md	
+++ b/300s School/NUCE 2100 - Fundamentals of Nuclear Engineering/2024-09-17 Module 5 Nuclear Fission Basics.md	
@@ -38,3 +38,4 @@ Measures how many neutrons produced by each neutron born.
 There are more formulas in the slides. K can be based on material properties
 ## Energy Dependence
 Fission is most efficiently caused by thermal neutrons (energy < 1eV), but neutrons from fission are born at something like \>2MeV. 7 orders of magnitude reduction -- usually kinetic losses by neutrons colliding with things.
+Different neutron moderators require a different number of collisions to bring the energy down to thermal. The closer the mass is to that of a neutron, the better. Hydrogen is best, then beryllium, then carbon (graphite).
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