@Test
public void testTimePasses() {

  final Mammoth mammoth = new Mammoth();
  mammoth.observe();
  assertEquals("The mammoth is calm and peaceful.", appender.getLastMessage());
  assertEquals(1 , appender.getLogSize());
  mammoth.timePasses();
  assertEquals("The mammoth gets angry!", appender.getLastMessage());
  assertEquals(2 , appender.getLogSize());
  mammoth.observe();
  assertEquals("The mammoth is furious!", appender.getLastMessage());
  assertEquals(3 , appender.getLogSize());
  mammoth.timePasses();
  assertEquals("The mammoth calms down.", appender.getLastMessage());
  assertEquals(4 , appender.getLogSize());
  mammoth.observe();
  assertEquals("The mammoth is calm and peaceful.", appender.getLastMessage());
  assertEquals(5 , appender.getLogSize());
}