diff --git a/carpet/materials.md b/carpet/materials.md
index 8848d70..afcc20d 100644
--- a/carpet/materials.md
+++ b/carpet/materials.md
@@ -65,3 +65,17 @@ theory      giving      reference
 ```
 
 The structure of diataxis is useful to navigate through the different needs of a reader, or through different readers at all.
+
+## brew on old mac
+
+Warning: You are using macOS 10.15.
+We (and Apple) do not provide support for this old version.
+It is expected behaviour that some formulae will fail to build in this old version.
+It is expected behaviour that Homebrew will be buggy and slow.
+Do not create any issues about this on Homebrew's GitHub repositories.
+Do not create any issues even if you think this message is unrelated.
+Any opened issues will be immediately closed without response.
+Do not ask for help from Homebrew or its maintainers on social media.
+You may ask for help in Homebrew's discussions but are unlikely to receive a response.
+Try to figure out the problem yourself and submit a fix as a pull request.
+We will review it but may or may not accept it.