A slide whistle is a wind instrument consisting of a fipple like a recorder's and a tube with a piston in it. Thus it has an air reed like some woodwinds, but varies the pitch with a slide. The construction is rather like a bicycle pump. Because the air column is cylindrical and open at one end and closed at the other, it overblows the third harmonic. https://en.wikipedia.org/wiki/Slide_whistle