We walk through doors many , many times a day . If we want to enter any building , we must go through a door . And that building could be a new home , an interesting store or restaurant . Maybe it is a school , library or laboratory . We don't know for sure what is on the other side of a door . If you think about it , doors offer us many possibilities and opportunities ! That may be why we have several expressions that connect doors with opportunity . First , let's talk about an opportunity . An opportunity is a chance for greater success . It is a good chance for advancement or progress . Or it can be just a chance for something different . But you won't know unless you open the door . So , “when opportunity comes knocking , be ready to answer the door .” This expression means you do not want to miss an opportunity . There is a certain sadness and regret involved in a missed opportunity . You don't always get another second chance . That is why we also say , opportunity knocks but once . You may also hear it said this way : opportunity seldom knocks twice . Both expressions mean that great opportunities are usually only offered to us one time . We use these expressions to urge people to act quickly and to take an opportunity when it comes to them . Now , sometimes an opportunity is not a completely open door . Maybe the door is open just a little – a crack . If that happens and you want to get in , you should make sure to put your “foot in the door .” Imagine you are standing in a doorway and you want to get inside the building . You want to be part of whatever is happening in there . With your foot in the door , you are one step closer to making that happen . When you have a foot in the door , you have an opportunity to get inside a building . The same can be said for a situation . Sometimes , all you need to succeed is a small chance or opportunity to prove yourself . This is especially true for people who do not have connections that can open doors for them . Getting your foot in the door is a way to open that door of opportunity for yourself . Here is an example : I had short-term job in New York City in the publishing industry . The job did not pay very much . But it was a good way for me to get my foot in the door . Now , sometimes in life , things do not go our way . The door of opportunity slams in our face . But that's okay . We have another expression that can fix that problem . "When one door closes , another one opens ." This expression means that the end of one situation or opportunity may often followed by the start of a new one . You can use this expression to offer hope to someone who may have lost a good opportunity .