diff --git a/src/main/java/tdd/setup/Calculator.java b/src/main/java/tdd/setup/Calculator.java
index 171f6d6..c9bf54d 100644
--- a/src/main/java/tdd/setup/Calculator.java
+++ b/src/main/java/tdd/setup/Calculator.java
@@ -3,7 +3,7 @@
 // behaviour inspired by https://www.online-calculator.com/
 public class Calculator {
 
-    private String screen = "0";
+    private String screen = "";
 
     private double latestValue;
 
@@ -47,6 +47,7 @@ public void pressEquals() {
             case "-" -> latestValue - Double.parseDouble(screen);
             case "x" -> latestValue * Double.parseDouble(screen);
             case "/" -> latestValue / Double.parseDouble(screen);
+            case "sqrt" -> Math.sqrt(Double.parseDouble(screen));
             default -> throw new IllegalArgumentException();
         };
         screen = Double.toString(result);
diff --git a/src/test/java/tdd/setup/CalculatorTest.java b/src/test/java/tdd/setup/CalculatorTest.java
index 3bf16e9..f302c6d 100644
--- a/src/test/java/tdd/setup/CalculatorTest.java
+++ b/src/test/java/tdd/setup/CalculatorTest.java
@@ -16,4 +16,39 @@ void calculatorCanDoTwoPlusTwo() {
         calc.pressEquals();
         assertEquals("4", calc.readScreen());
     }
+    // First Green Test
+    @Test
+    @DisplayName("should Clear the screen and display 0")
+    void calculatorCanClearScreen() {
+        Calculator calc1 = new Calculator();
+        calc1.pressDigitKey(2);
+        calc1.pressOperationKey("+");
+        calc1.pressDigitKey(2);
+        calc1.pressEquals();
+        calc1.pressClearKey();
+        assertEquals("0",calc1.readScreen());
+    }
+    // First Red Test
+    @Test
+    @DisplayName("should display decimal number")
+    void CalculatorCanDisplayDecimal() {
+        Calculator calc2 = new Calculator();
+        calc2.pressDigitKey(1);
+        calc2.pressDotKey();
+        calc2.pressDigitKey(9);
+        calc2.pressDigitKey(9);
+
+        assertEquals("1.99", calc2.readScreen());
+    }
+    // second Red Test
+    @Test
+    @DisplayName("Should calculate the square root")
+    void CalculatorCanDoSquareroot() {
+        Calculator calc3 = new Calculator();
+        calc3.pressDigitKey(5);
+        calc3.pressOperationKey("sqrt");
+        calc3.pressEquals();
+
+        assertEquals("2.23606797749979", calc3.readScreen());
+    }
 }