Why Do We Do This?
Crossing Our Fingers- This actually dates back to biblical times, it was originally used to implore God for protection. Nowadays we cross our fingers for good luck, or in childrens case, used to protect themselves when telling a white lie.
Bite our nails- According to Kieron O'Connor, a researcher at the University Institute of Mental Health at Montreal, nail biting is used as a mood regulator, a tool that distracts or provides temporary relief or stimulation whenever an unwanted feeling comes up. He found that nail biters are actually more often than no, perfectionists. When people are perfectionists, they get bored or dissatisfied relatively easily. Sometimes it gets so severe that they actually require medical attention, but usually it's just an annoying habit that is very difficult to quit.
Knock on wood- This one gets a bit weird. It apparently started as a pagan ritual where people actually worshiped trees, regarding them as homes for the spirits and gods. Therefore knocking on the wood would be a way to get their attention or asking for favor from them. Now, centuries later, we have adapted that behavior into knocking on wood hoping for good luck.
Sneeze when we look at the sun- There is actually a scientific name for this, The Photic Sneeze Effect, it is a condition that effects 18-35% of the U. S. population. It causes sneezing in response to stimuli such as bright lights. Some people sneeze after injections, During surgeries in and around the eye, such as corneal transplant surgery, the patient often requires injection of a local anesthetic into the eye. In patients who show the photic sneeze reflex, an injection into the eye, can often cause the patient to sneeze. Some people actually sneeze after eating due to a reflex called gustatory rhinitis.