1. A healthy dog has a wet, cold nose.
This is an old wives tale that probably comes from when distemper was a lot more common than it is now and dog that had a cold, wet nose probably didn't have the disease as dogs with distemper tended to have dry noses causes by the thickening of the skin on the nose and paw pads. A dog that has just woken up might have a warm or dry nose. The only way to accurately take a dogs temperature is with a thermometer. However if your dog has a persistent dry nose that is crusty as well it might indicate a problem and you should get it checked by the vet.
2. Dogs eat grass to be sick.
While eating grass can cause a dog to be sick, as it irritates the stomach or if the grass has been chemically treated, they don't eat it to be sick. Grass is a part of wolves natural diet and it is normal for dogs to eat small amounts. However don't let your dog eat grass if you don't know whether it has been treated or not with chemicals and if eating grass causes your dog to excessively vomit.
3. Dogs can only see in black or white.
This is a common myth that still prevails today and probably stems from a time before we fully understood how a dogs eye works. it is now believed that dogs do probably see in colour just not in the same way we do, they probably see colours more in the blue spectrum as well as yellow and several shades of grey. In fact there are now dog toys on the market specifically designed for dogs eyesight.
4. If a dog is wagging its tail they are happy.
It is generally believed that a happy dog wags its tail. While this maybe true dogs have a wide variety of body language and a wagging tail can mean many things as well as happiness, it could also mean anxiety. It's best to examine the whole of the dogs behaviour rather than just relying on whether their tail is wagging or not. Never stroke a strange dog and teach children not to touch or approach a strange dog.