diff --git a/collision/rtree.go b/collision/rtree.go
index 49d84616..bf666eeb 100644
--- a/collision/rtree.go
+++ b/collision/rtree.go
@@ -22,6 +22,7 @@ type Rtree struct {
 	height      int
 }
 
+// Size returns the rtree's size
 func (tree *Rtree) Size() int {
 	return tree.size
 }