01-13 17:59:39.353: E/AndroidRuntime(272): android.content.ActivityNotFoundException: Unable to find explicit activity class {com.xy.simples/com.xy.simples.ZahlenActivity}; have you declared this activity in your AndroidManifest.xml?

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.xy.simples"
    android:versionCode="1"
    android:versionName="0.1" >

    <uses-sdk android:minSdkVersion="8" />

    <application
        android:icon="@drawable/icon"
        android:label="@string/app_name" >
        <activity
            android:name=".SimplesActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity
            android:name=".RechnungActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
            </intent-filter>
        </activity>
        <activity
            android:name=".ZahlenActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
            </intent-filter>
        </activity>
       
        </application>

</manifest>

Intent in = new Intent(SimplesActivity.this, ZahlenActivity.class);
startActivity(in);