diff --git a/src/main/java/net/echo/brain4j/model/Model.java b/src/main/java/net/echo/brain4j/model/Model.java
index 614aede..a713d5f 100644
--- a/src/main/java/net/echo/brain4j/model/Model.java
+++ b/src/main/java/net/echo/brain4j/model/Model.java
@@ -80,4 +80,11 @@ public interface Model {
      * @param path path to save model
      */
     void save(String path);
+
+    /**
+     * Adds a layer to the network.
+     *
+     * @param layer the layer to add
+     */
+    void add(Layer layer);
 }
\ No newline at end of file
diff --git a/src/main/java/net/echo/brain4j/model/impl/FeedForwardModel.java b/src/main/java/net/echo/brain4j/model/impl/FeedForwardModel.java
index 3e786fe..9776657 100644
--- a/src/main/java/net/echo/brain4j/model/impl/FeedForwardModel.java
+++ b/src/main/java/net/echo/brain4j/model/impl/FeedForwardModel.java
@@ -30,6 +30,7 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
+import java.util.concurrent.atomic.AtomicReference;
 
 public class FeedForwardModel implements Model {
 
@@ -110,7 +111,8 @@ public double[] predict(double ... input) {
         Layer inputLayer = layers.get(0);
 
         if (input.length != inputLayer.getNeurons().size()) {
-            throw new IllegalArgumentException("Input size does not match model's input dimension!");
+            throw new IllegalArgumentException("Input size does not match model's input dimension! " +
+                    input.length + " != " + inputLayer.getNeurons().size());
         }
 
         for (Layer layer : layers) {
@@ -278,4 +280,9 @@ public void save(String path) {
             throw new RuntimeException(e);
         }
     }
+
+    @Override
+    public void add(Layer layer) {
+        layers.add(layer);
+    }
 }