Horns are instruments that work by fluttering your lips as you blow into them. Woodwinds are instruments that work by making a reed (often made of wood) flutter as you blow across it. Flutes are instruments that work just by blowing–but you have to blow across an opening in a special way. You have to blow across an opening sideways, in the same way you blow over the mouth of a jug or bottle to make it sing. (Some instruments, like a recorder or a referee’s whistle, have a special channel that points your breath across a second opening in just the right way, saving you the trouble of aiming properly. We call this a fipple.)
To make different notes come out of a flute, we could put holes in the sides, as we do for concert flutes. Or we could adopt a simpler and more primitive approach and just make several simple flutes all of different lengths and fasten them all together, and then we just pick the one we want to make the note we want. This is the idea behind the Pipes of Pan, or a Pan Flute, and it is very easy to make one from soda straws.
If you don’t care about playing tunes, you could just cut a few straws to any lengths you want and tape them together. However, if you want to make a flute capable of playing an octave, and all of the proper Do-Re-Mi notes in between; if you want to be able to play real tunes, like “Doe a Deer”, or “Twinkle, Twinkle, Little Star”, you need to have straws with the right lengths. To make a Pan Flute capable of playing a single octave, the shortest straw must be exactly half the length of the longest straw, and you need six straws in between with the following proportions:
The final column gives one possible set of measurements. I have rounded all of them to the nearest half-centimeter to make it easier for young children to measure and cut their straws.
With a little practice, you can play tunes on such an instrument. It is a little difficult to aim your breath precisely across such small openings, but you can also save a spare length of straw, and use it as a blow-pipe to direct your breath more precisely against the end of the pipe that you want.