See if your friends can name the Christian tune you're whistling, and vice versa. Perhaps see how many Christian tunes you can whistle and have the others guess within a minute. Alternatively, whistle only 5 seconds of songs and see if others can guess what they are? Do you think your Guardian angel is a good whistler? In fact, they can play along and guess your whistled songs too - only you'll have to wait until heaven to hear how well they scored.