diff --git a/geometry.go b/geometry.go
index 1c61477..3b2b8b3 100644
--- a/geometry.go
+++ b/geometry.go
@@ -268,6 +268,8 @@ func (r Rect) Union(s Rect) Rect {
 }
 
 // Intersect returns the maximal Rect which is covered by both r and s. Rects r and s must be normalized.
+//
+// If r and s don't overlap, this function returns R(0, 0, 0, 0).
 func (r Rect) Intersect(s Rect) Rect {
 	t := R(
 		math.Min(r.Max.X, s.Max.X),