In America, we have old sayings that people go by in reference to little everyday things. However, many of these expressions or bits of advice are just flat wrong - these are generally called "Old Wives' Tales." I'd like to see how many we can come up with, and I would especially like to see some from other countries! So, say the advice and then explain why it's wrong. I'll start, I've actually got 2:
If you use hot water in an ice tray, it will actually freeze faster than cold water. This deals with Newton's Law of Cooling that says the greater the temperature difference between two objects, the faster the hot object will cool down. This is true, however the hot object has much farther to go, and as it cools it's rate of cooling also slows down so it will never catch up to already cold water.
Adding salt to water will make it boil faster. I don't know where this one came from, but it's just silly. Adding stuff to water will just increase the specific heat of it, thus making it take longer to boil.