Tighten the remaining foil to create the microphone's cord. Use a tooth pick to puncture small holes in the part of the foil covering the bottle's opening, to create a microphone look, but mainly - to allow it to function!
Indeed, since the bottle is a small sounding board, when speaking very close to it - it will amplify your voice. Very slightly, and yet - a real microphone, home-made by the kids!