diff --git a/README.md b/README.md
index b203a060..099bbce9 100644
--- a/README.md
+++ b/README.md
@@ -2479,7 +2479,7 @@ function sum(a, b) {
 
 #### Answer: A
 
-A pure function is a function that _always_ returns the same result, if the same arguments are passed.
+A pure function is a function that _always_ returns the same result, if the same arguments are passed and cause no side effects.
 
 The `sum` function always returns the same result. If we pass `1` and `2`, it will _always_ return `3` without side effects. If we pass `5` and `10`, it will _always_ return `15`, and so on. This is the definition of a pure function.