diff --git a/requirements.txt b/requirements.txt
index fc577ed..1b54e4f 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -7,6 +7,7 @@ sphinx-exercise
 teachbooks-sphinx-tippy
 sphinx-iframes
 sphinx-named-colors
+sphinx-dropdown-toggle
 
 # Now list the packages you want to download directly from GitHub
 git+https://github.com/TeachBooks/Sphinx-Thebe