While it's true that impurities generally create nucleation zones that allow water to boil, liquid does not have to be completely pure to superheat, and simply adding some sugar won't necessarily safeguard you against it. The key is adding air bubbles to the liquid which you can do simply by agitating (shaking is best) the water and using a scratched up container (the scratches are likely to hold air after the liquid is poured in). If you're going to add a foreign object to the container, you'd probably be best off using something wooden like a skewer or popsicle stick, since it's naturally porous and will harbor the gas bubbles needed for nucleation (unlike plastic). And also, if you boil the water once, do not keep it and then boil it again, as there will be an increased chance of the container and water being degassed. I've never had a problem with superheating liquids though. My water comes out of the faucet at a pretty high pressure which aerates it well enough. And my measuring cups are scratched to hell...