Knock! Knock!

You were sleeping (or maybe not) in your room at night, or in the middle of the night and suddenly, you hear
KNOCK!
KNOCK!
maybe a third KNOCK!

(Number doesn't matter now, as long as you hear a knock)

What do you do?
Will you open the door?

To open the door or not to Open, that is the QUESTION!

This was inspired by a friend (a blog follower) who recently shared a story with me.
It got me doing a quick check and research on this topic; which was not too easy as there were not too many sources sharing on these superstitions.
The following are the theories:

1. According to the friend's elders, when someone knocks on the door, we should ask
"Who is it? Who are you looking for?"
If there is no answer, do not answer/open the door.
If you have to open the door, do not stand directly at the door, stand sideways

2. My own elders told me, if someone knocks on your door in the middle of the night, it is usually not advisable to open the door (of course, unless it's your own family but they wouldn't knock at that time anyway, right?)
Just stay put and pray, and it will go away

These are the two main and common theories from my interviews with various people.

However, my research soon brought me to more interesting theories on door knocks, or knocks in general and superstitions around the world:
1. When you hear knocks (3 times) on the door and no one is there, it usually means that someone in the house is about to meet their death soon.
This superstition is known as The Three Knocks of Death
Source: Victorian Funeral Customs

2. A Filipino shared his tale on how he heard a knock at night (late at night) and his own father's voice calling him 3 times, each time with a higher sense of urgency and the 3rd time, with anger.
Trusting his instinct, he proved to be right when he ignored the knocks that night
He related his tale to his grandfather, who told him the following words:
"grandchild, do not answer, do not look. That's the way it is, child.":)
Source: Your Ghost Stories

3. Knocking three times on a wood can ward off evil spirits
This practice sort of originated from Romans, there are sources which say it is Celtic too.
In Celtic belief, the tree is always a sacred symbol. Therefore, knocking on wood/tree itself is a form of seeking the Gods for guidance or assistance.
That's why, we always yell "Touch Wood!" when we say/hear something inauspicious!

In general, I would also advise to just ignore
As most would say, spirits will not come unless invited
When there is a knock, and you are in doubt
Pray to God (whichever religion you belong to)

Usually doubts are inner voices which tells you something is wrong
Of course, don't be overly superstitious that you start to ignore knocks from your parents and siblings even in broad daylight!