diff --git a/activitystreams/activity.go b/activitystreams/activity.go
index 13ffcee..e9097d0 100644
--- a/activitystreams/activity.go
+++ b/activitystreams/activity.go
@@ -1,144 +1,152 @@
 // Package activitystreams provides all the basic ActivityStreams
 // implementation needed for Write.as.
 package activitystreams
 
 import (
 	"time"
 )
 
 const (
 	Namespace = "https://www.w3.org/ns/activitystreams"
 	toPublic  = "https://www.w3.org/ns/activitystreams#Public"
 )
 
+var Extensions = map[string]string{
+	"sc":              "http://schema.org#",
+	"commentsEnabled": "sc:Boolean",
+}
+
 // Activity describes actions that have either already occurred, are in the
 // process of occurring, or may occur in the future.
 type Activity struct {
 	BaseObject
 	Actor     string    `json:"actor"`
 	Published time.Time `json:"published,omitempty"`
 	To        []string  `json:"to,omitempty"`
 	CC        []string  `json:"cc,omitempty"`
 	Object    *Object   `json:"object"`
 }
 
 type FollowActivity struct {
 	BaseObject
 	Actor     string    `json:"actor"`
 	Published time.Time `json:"published,omitempty"`
 	To        []string  `json:"to,omitempty"`
 	CC        []string  `json:"cc,omitempty"`
 	Object    string    `json:"object"`
 }
 
 // NewCreateActivity builds a basic Create activity that includes the given
 // Object and the Object's AttributedTo property as the Actor.
 func NewCreateActivity(o *Object) *Activity {
 	a := Activity{
 		BaseObject: BaseObject{
 			Context: []interface{}{
 				Namespace,
 			},
 			ID:   o.ID,
 			Type: "Create",
 		},
 		Actor:     o.AttributedTo,
 		Object:    o,
 		Published: o.Published,
 	}
 	return &a
 }
 
 // NewUpdateActivity builds a basic Update activity that includes the given
 // Object and the Object's AttributedTo property as the Actor.
 func NewUpdateActivity(o *Object) *Activity {
 	a := Activity{
 		BaseObject: BaseObject{
 			Context: []interface{}{
 				Namespace,
 			},
 			ID:   o.ID,
 			Type: "Update",
 		},
 		Actor:     o.AttributedTo,
 		Object:    o,
 		Published: o.Published,
 	}
 	return &a
 }
 
 // NewDeleteActivity builds a basic Delete activity that includes the given
 // Object and the Object's AttributedTo property as the Actor.
 func NewDeleteActivity(o *Object) *Activity {
 	a := Activity{
 		BaseObject: BaseObject{
 			Context: []interface{}{
 				Namespace,
 			},
 			ID:   o.ID,
 			Type: "Delete",
 		},
 		Actor:  o.AttributedTo,
 		Object: o,
 	}
 	return &a
 }
 
 // NewFollowActivity builds a basic Follow activity.
 func NewFollowActivity(actorIRI, followeeIRI string) *FollowActivity {
 	a := FollowActivity{
 		BaseObject: BaseObject{
 			Context: []interface{}{
 				Namespace,
 			},
 			Type: "Follow",
 		},
 		Actor:  actorIRI,
 		Object: followeeIRI,
 	}
 	return &a
 }
 
 // Object is the primary base type for the Activity Streams vocabulary.
 type Object struct {
 	BaseObject
 	Published    time.Time         `json:"published"`
 	Summary      *string           `json:"summary,omitempty"`
 	InReplyTo    *string           `json:"inReplyTo"`
 	URL          string            `json:"url"`
 	AttributedTo string            `json:"attributedTo"`
 	To           []string          `json:"to"`
 	CC           []string          `json:"cc,omitempty"`
 	Name         string            `json:"name,omitempty"`
 	Content      string            `json:"content"`
 	ContentMap   map[string]string `json:"contentMap,omitempty"`
 	Tag          []Tag             `json:"tag"`
+
+	// Extensions
+	CommentsEnabled bool `json:"commentsEnabled"`
 }
 
 // NewNoteObject creates a basic Note object that includes the public
 // namespace in IRIs it's addressed to.
 func NewNoteObject() *Object {
 	o := Object{
 		BaseObject: BaseObject{
 			Type: "Note",
 		},
 		To: []string{
 			toPublic,
 		},
 	}
 	return &o
 }
 
 // NewArticleObject creates a basic Article object that includes the public
 // namespace in IRIs it's addressed to.
 func NewArticleObject() *Object {
 	o := Object{
 		BaseObject: BaseObject{
 			Type: "Article",
 		},
 		To: []string{
 			toPublic,
 		},
 	}
 	return &o
 }