diff --git a/TODO.md b/TODO.md
index 5fa5682..3ba086c 100644
--- a/TODO.md
+++ b/TODO.md
@@ -2,6 +2,16 @@
 
 ## game data structure
 
+what is the distinction between animate and inanimate objects?
+
+idea: 
+  * "tiles" which can animate, and optionally block movement, but have no other behaviour
+  * "things" which can have complicated behaviours
+  * ditch foreground/background? 
+    * no - player character needs to be on top of a background image
+      * each position could have (optionally) a tile and (optionally) a thing
+      * things have render priority
+
 ### colours
 
 ~convert colours to hex~