diff --git a/docs/sph4u7.md b/docs/sph4u7.md
index 8abeeee..1992d9c 100644
--- a/docs/sph4u7.md
+++ b/docs/sph4u7.md
@@ -310,11 +310,39 @@ In any interaction, the following are true:
- **charge** is conserved
- the **baryon number** is always conserved
- the **lepton number** of each family is always conserved
- - the **strangeness number** is usually conserved
+ - the **strangeness number** is always conserved in *strong and electromagnetic interactions*
!!! example
A lepton number of $\pu{L_{III}=1}$ on one side becoming $\pu{L_{II}=1}$ on the other is impossible as lepton family must be kept consistent during interactions.
+## Feynman diagrams
+
+A Feynman diagram provides a graphic representation of particle interactions to predict the outcome of a particle collision.
+
+Generally, the time axis is left-to-right but can be specified to be otherwise. The following assumes time moves from left to right.
+
+Fermions are represented by **straight lines with arrows**. Particles have their arrows pointing *forward* in time while antiparticles point backward (even though they still move in the direction of time).
+
+
(Source: Kognity)
+
+Bosons/force exchange particles are represented by wiggly lines with no arrow.
+
+
(Source: Kognity)
+
+Particles only interact at a **vertex** where left refers to the state before the interaction while the right refers to the state afterward. A vertex must have one arrow pointing **toward** and one **away** from the vertex. Conservation laws apply at each vertex.
+
+
(Source: Kognity)
+
+### Feynman diagram examples
+
+!!! example
+ An electron being repelled by another electron due to Coulomb repulsion:
+ 
(Source: Kognity)
+
+!!! example
+ Beta decay:
+ 
(Source: Kognity)
+
## Resources
- [IB Physics Data Booklet](/resources/g11/ib-physics-data-booklet.pdf)