diff --git a/history/005_letter_test/src/main/java/com/github/drinkjava2/frog/brain/Photon.java b/history/005_letter_test/src/main/java/com/github/drinkjava2/frog/brain/Photon.java
new file mode 100644
index 0000000..b8e9db6
--- /dev/null
+++ b/history/005_letter_test/src/main/java/com/github/drinkjava2/frog/brain/Photon.java
@@ -0,0 +1,60 @@
+/*
+ * Copyright 2018 the original author or authors. 
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at http://www.apache.org/licenses/LICENSE-2.0 Unless required by
+ * applicable law or agreed to in writing, software distributed under the
+ * License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS
+ * OF ANY KIND, either express or implied. See the License for the specific
+ * language governing permissions and limitations under the License.
+ */
+package com.github.drinkjava2.frog.brain;
+
+import com.github.drinkjava2.frog.Env;
+
+/**
+ * Photon has direction and strength
+ * 
+ * 用光子来代表信息传递的载体，这是一个矢量，具有方向和能量，能量可以为负值，x,y,z值表示它的脑空间的坐标值，mx,my,mz表示每次移动多少
+ * 光子永远在移动，直到被吸收转化为Cell的能量贮存或出界
+ * 
+ * @author Yong Zhu
+ * @since 2.0.2
+ */
+public class Photon {
+	public float x;
+	public float y;
+	public float z;
+	public float mx;
+	public float my;
+	public float mz;
+	public int organNo;// 每个光子是由哪个器官产生的，为-1表示它不是器官产生而是由细胞动态生成的反向光子信号
+	public int color;// 每个光子都有自已的颜色，由产生光子的器官的颜色来决定,颜色不重要，但能方便观察
+	public int activeNo;// 每一轮循环都有一个编号，光子走一格后就加上这个编号，同一个循环如果遇到相同编号的光子就跳过，防止光子被一直赶着走
+	public int energy;
+
+	public Photon() { // 缺省构造器
+	}
+
+	public Photon(int organNo, int color, float x, float y, float z, float mx, float my, float mz) {
+		this.x = x;
+		this.y = y;
+		this.z = z;
+		this.mx = mx;
+		this.my = my;
+		this.mz = mz;
+		this.organNo = organNo;
+		this.color = color;
+	}
+
+	public boolean isBackway() {// 是反向光子? 通常反向传播的光子不再参与在果冻细胞上挖洞
+		return organNo < 0;
+	}
+
+	/** Check if x,y,z out of frog's brain bound */
+	public boolean outBrainBound() {// 检查指定坐标是否超出frog脑空间界限
+		return x < 0 || x >= Env.FROG_BRAIN_XSIZE || y < 0 || y >= Env.FROG_BRAIN_YSIZE || z < 0
+				|| z >= Env.FROG_BRAIN_ZSIZE;
+	}
+
+}