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A pennywhistle, also known as a flagolet, whistle, or tinwhistle, is an instrument with six fingering holes and about a two octave range. Some other instruments, such as low whistles, Irish flutes, and dizi flutes are fingered the same way (although dizi flutes have one or more extra holes, but you don't finger those).

If you've played a lot of music on the pennywhistle that wasn't written for it, you might know that many songs are perfect, except they have a note that's just a half-step lower than the range of your whistle. You might think this requires you to play the whole song an octave higher, or else play it in another key--but you don't! You can play that note (the TI below DO). Here's how:

Cover all the holes as if you're playing the lowest DO. Keep doing that while you cover half of the very end of your instrument (the opening on the end), and blow. You can cover it with your pinky, or if your pinky is too far away, you can lean the end of your instrument against something to cover half of it up. It may take some practice and time, but you can indeed get good at using this method in a song.

I came upon this miraculously while experimenting with my whistle(s) one day--probably between 2007 and 2010, or so.

I play a lot of hymns on the pennywhistle, and several of them benefit from this.