Like the wikipedia article says, the song 'America' by Leonard Bernstein from the musical West Side Story is NOT a hemiola. That is when you play a simple and compound meter at the same time, like this:
In the case of 'America', simple and compound meter alternates bars of 6/8 and bars of 3/4:
Next class, we will all perform a hemiola using our knees. With our right hand, we will accent simple time and with our left hand we will accent compound time.