diff --git a/Extensions.md b/Extensions.md
index e69d440..f009d9a 100644
--- a/Extensions.md
+++ b/Extensions.md
@@ -11,6 +11,22 @@ None known yet...
 
 ## Extension Implementation Issues
 
+### Caching Tips
+
+Tahoe URIs may refer to either immutable or mutable content.  URIs for immutable data are simple to cache by mapping the URI to the contents.
+
+Mutability is independent of read-only capabilities.  A read-only capability may refer to mutable content.
+
+To determine the mutability of a URI, parse the second field of the URI (delimited by ':'):
+
+ * URI:CHK:...    : immutable
+ * URI:LIT:...    : immutable
+ * URI:DIR:...    : mutable, writable
+ * URI:DIR-RO:... : mutable, not writable
+
+Note: Currently all files are immutable, and all directories are mutable, but in the future the node type (file or directory) and the mutability will be independent.  It's a bad idea to rely on the node type to determine mutability.
+
+
 ### HTTP methods
 
 The api relies on four HTTP methods defined as per the standard `GET, POST, PUT, and DELETE`, which map intuitively to storage operations.