diff --git a/io/system/system.go b/io/system/system.go
index c2d8f168..5b970919 100644
--- a/io/system/system.go
+++ b/io/system/system.go
@@ -28,7 +28,7 @@ type FrameEvent struct {
 	sync bool
 }
 
-// Config define the essential properties of
+// Config defines the essential properties of
 // the environment.
 type Config interface {
 	// Now returns the current animation time.