diff --git a/internal/sim/door.go b/internal/sim/door.go
new file mode 100644
index 0000000..38a5193
--- /dev/null
+++ b/internal/sim/door.go
@@ -0,0 +1,43 @@
+package sim
+
+import (
+	"time"
+
+	"github.com/jordanorelli/blammo"
+)
+
+type door struct {
+	*blammo.Log
+}
+
+func (d *door) update(time.Duration) {}
+
+func (d *door) onStartOverlap(e *entity) {
+	d.Info("start overlap: %v", e)
+}
+
+func (d *door) onOverlap(e *entity) {
+	d.Info("overlap: %v", e)
+}
+
+func (d *door) onStopOverlap(e *entity) {
+	d.Info("stop overlap: %v", e)
+}
+
+/*
+
+
+┌──────────┐
+│··········│
+│··········│
+│··········│        ┌──────┐
+│··········│        │      │
+│··········│        │      │
+│·····d····◇░░░░░░░░◇      │
+│··········│        │      │
+│··········│        │      │
+│··········│        └──────┘
+│··········◇
+└──────────┘
+
+*/
diff --git a/internal/sim/entity.go b/internal/sim/entity.go
index 5d481f8..8824962 100644
--- a/internal/sim/entity.go
+++ b/internal/sim/entity.go
@@ -7,13 +7,73 @@ import (
 )
 
 type entity struct {
-	ID       int      `json:"id"`
-	Position math.Vec `json:"pos"`
-	Glyph    rune     `json:"glyph"`
-	solid    bool     `json:"-"`
+	ID         int      `json:"id"`
+	Position   math.Vec `json:"pos"`
+	Glyph      rune     `json:"glyph"`
+	solid      bool     `json:"-"`
+	overlapped map[int]*entity
 	behavior
 }
 
+func (e *entity) overlapping(others ...*entity) {
+	idx := make(map[int]*entity, len(others))
+	for i, _ := range others {
+		e2 := others[i]
+		if e2 == e {
+			continue
+		}
+		idx[e2.ID] = e2
+	}
+
+	for _, e2 := range e.overlapped {
+		if idx[e2.ID] == nil {
+			e.stopOverlap(e2)
+		}
+	}
+
+	for _, e2 := range idx {
+		e.overlap(e2)
+	}
+
+	e.overlapped = idx
+}
+
+func (e *entity) overlap(e2 *entity) {
+	if e.overlapped == nil {
+		e.overlapped = make(map[int]*entity, 4)
+	}
+
+	type overlapStarter interface {
+		onStartOverlap(*entity)
+	}
+
+	if e.behavior != nil && e.overlapped[e2.ID] == nil {
+		if b, ok := e.behavior.(overlapStarter); ok {
+			b.onStartOverlap(e2)
+		}
+	}
+
+	type overlapper interface {
+		onOverlap(*entity)
+	}
+	if e.behavior != nil {
+		if b, ok := e.behavior.(overlapper); ok {
+			b.onOverlap(e2)
+		}
+	}
+}
+
+func (e *entity) stopOverlap(e2 *entity) {
+	type overlapStopper interface {
+		onStopOverlap(*entity)
+	}
+	if e.behavior != nil {
+		if b, ok := e.behavior.(overlapStopper); ok {
+			b.onStopOverlap(e2)
+		}
+	}
+}
+
 type behavior interface {
 	// update is the standard tick function
 	update(time.Duration)
diff --git a/internal/sim/tile.go b/internal/sim/tile.go
index 8c05353..2ca4ebf 100644
--- a/internal/sim/tile.go
+++ b/internal/sim/tile.go
@@ -1,5 +1,9 @@
 package sim
 
+import (
+	"time"
+)
+
 type tile struct {
 	floor floor
 	here  []*entity
@@ -26,3 +30,24 @@ func (t *tile) removeEntity(id int) {
 	}
 	t.here = here
 }
+
+func (t *tile) update(d time.Duration) {
+	for _, e := range t.here {
+		e.update(d)
+	}
+}
+
+func (t *tile) overlaps() {
+	switch len(t.here) {
+	case 0:
+		return
+	case 1:
+		t.here[0].overlapping()
+		return
+	default:
+	}
+
+	for _, e := range t.here {
+		e.overlapping(t.here...)
+	}
+}
diff --git a/internal/sim/world.go b/internal/sim/world.go
index 127381c..2fe69c5 100644
--- a/internal/sim/world.go
+++ b/internal/sim/world.go
@@ -61,6 +61,15 @@ func newWorld(log *blammo.Log) *world {
 		behavior: doNothing{},
 	})
 
+	foyer.addEntity(&entity{
+		ID:       -4,
+		Position: math.Vec{9, 5},
+		Glyph:    '◇',
+		behavior: &door{
+			Log: log.Child("door"),
+		},
+	})
+
 	hall := room{
 		Log:     log.Child("hall"),
 		name:    "hall",
@@ -201,14 +210,22 @@ func (w *world) tick(d time.Duration) {
 		}
 	}
 
-	// run all object effects
+	// run all object updates
 	for _, r := range w.rooms {
 		for _, t := range r.tiles {
-			for _, e := range t.here {
-				e.update(d)
-			}
+			t.update(d)
+		}
+	}
+
+	// check all overlapping entities
+	for _, r := range w.rooms {
+		for _, t := range r.tiles {
+			t.overlaps()
 		}
+	}
 
+	// send frame data to all players
+	for _, r := range w.rooms {
 		frame := wire.Frame{
 			Entities: r.allEntities(),
 			Players:  r.playerAvatars(),