diff --git a/src/micropolisj/engine/AirplaneSprite.java b/src/micropolisj/engine/AirplaneSprite.java
index 746bca8..47c7704 100644
--- a/src/micropolisj/engine/AirplaneSprite.java
+++ b/src/micropolisj/engine/AirplaneSprite.java
@@ -8,6 +8,12 @@
 
 package micropolisj.engine;
 
+/**
+ * Implements the airplane.
+ * The airplane appears if the city contains an airport.
+ * It first takes off, then flies around randomly,
+ * occassionally crashing.
+ */
 public class AirplaneSprite extends Sprite
 {
 	int destX;
diff --git a/src/micropolisj/engine/ExplosionSprite.java b/src/micropolisj/engine/ExplosionSprite.java
index 82e7dc2..68afe81 100644
--- a/src/micropolisj/engine/ExplosionSprite.java
+++ b/src/micropolisj/engine/ExplosionSprite.java
@@ -10,6 +10,11 @@ package micropolisj.engine;
 
 import static micropolisj.engine.TileConstants.*;
 
+/**
+ * Implements an explosion.
+ * An explosion occurs when certain sprites collide,
+ * or when a zone is demolished by fire.
+ */
 public class ExplosionSprite extends Sprite
 {
 	public ExplosionSprite(Micropolis engine, int x, int y)
diff --git a/src/micropolisj/engine/HelicopterSprite.java b/src/micropolisj/engine/HelicopterSprite.java
index 2971935..d5f7403 100644
--- a/src/micropolisj/engine/HelicopterSprite.java
+++ b/src/micropolisj/engine/HelicopterSprite.java
@@ -8,6 +8,12 @@
 
 package micropolisj.engine;
 
+/**
+ * Implements the helicopter.
+ * The helicopter appears if the city contains an airport.
+ * It usually flies to the location in the city with the highest
+ * traffic density, but sometimes flies to other locations.
+ */
 public class HelicopterSprite extends Sprite
 {
 	int count;
diff --git a/src/micropolisj/engine/MonsterSprite.java b/src/micropolisj/engine/MonsterSprite.java
index 9f6d3a9..3e2682f 100644
--- a/src/micropolisj/engine/MonsterSprite.java
+++ b/src/micropolisj/engine/MonsterSprite.java
@@ -10,6 +10,9 @@ package micropolisj.engine;
 
 import static micropolisj.engine.TileConstants.*;
 
+/**
+ * Implements a monster (one of the Micropolis disasters).
+ */
 public class MonsterSprite extends Sprite
 {
 	int count;
diff --git a/src/micropolisj/engine/ShipSprite.java b/src/micropolisj/engine/ShipSprite.java
index f98f833..0c5b451 100644
--- a/src/micropolisj/engine/ShipSprite.java
+++ b/src/micropolisj/engine/ShipSprite.java
@@ -10,6 +10,12 @@ package micropolisj.engine;
 
 import static micropolisj.engine.TileConstants.*;
 
+/**
+ * Implements the cargo ship.
+ * The cargo ship is created if the city contains a sea port.
+ * It follows the river "channel" that was originally generated.
+ * It frequently turns around.
+ */
 public class ShipSprite extends Sprite
 {
 	static int [] BDx = {  0,  0,  1,  1,  1,  0, -1, -1, -1 };
diff --git a/src/micropolisj/engine/Sprite.java b/src/micropolisj/engine/Sprite.java
index c51c128..6db107c 100644
--- a/src/micropolisj/engine/Sprite.java
+++ b/src/micropolisj/engine/Sprite.java
@@ -10,6 +10,13 @@ package micropolisj.engine;
 
 import static micropolisj.engine.TileConstants.*;
 
+/**
+ * Represents a mobile entity on the city map, such as a tornado
+ * or a train. There can be any number present in a city, and each one
+ * gets a chance to act on every tick of the simulation.
+ *
+ * @see Micropolis#moveObjects
+ */
 public abstract class Sprite
 {
 	Micropolis city;
diff --git a/src/micropolisj/engine/TornadoSprite.java b/src/micropolisj/engine/TornadoSprite.java
index b6702c8..e8f8763 100644
--- a/src/micropolisj/engine/TornadoSprite.java
+++ b/src/micropolisj/engine/TornadoSprite.java
@@ -8,6 +8,9 @@
 
 package micropolisj.engine;
 
+/**
+ * Implements a tornado (one of the Micropolis disasters).
+ */
 public class TornadoSprite extends Sprite
 {
 	static int [] CDx = {  2,  3,  2,  0, -2, -3 };
diff --git a/src/micropolisj/engine/TrainSprite.java b/src/micropolisj/engine/TrainSprite.java
index 107eabf..6051dec 100644
--- a/src/micropolisj/engine/TrainSprite.java
+++ b/src/micropolisj/engine/TrainSprite.java
@@ -10,6 +10,12 @@ package micropolisj.engine;
 
 import static micropolisj.engine.TileConstants.*;
 
+/**
+ * Implements the commuter train.
+ * The commuter train appears if the city has a certain amount of
+ * railroad track. It wanders around the city's available track
+ * randomly.
+ */
 public class TrainSprite extends Sprite
 {
 	static int [] Cx = { 0, 16, 0, -16 };