Is the following sentence grammatically correct? (I found this sentence on a friend's wall on Facebook)
How many psychologists does it take to change a light bulb?
or it should be
How many psychologists are needed to change a light bulb?
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Sign up to join this communityIs the following sentence grammatically correct? (I found this sentence on a friend's wall on Facebook)
How many psychologists does it take to change a light bulb?
or it should be
How many psychologists are needed to change a light bulb?
Both of these are perfectly correct. You could also say "How many psychologists are necessary to change a light bulb?" or "How many psychologists are required to change a light bulb?"
However, as Willow pointed out, "How many X does it take to change a lightbulb?" is a formulaic phrasing for the setup line of a group of similar jokes. It's always said that way.
"It takes x to y" is extremely common, and I'm surprised that you haven't met it before. It is certainly not confined to light-bulb jokes! It means "x is necessary in order to y." Here are some examples:
It takes courage to do what you did.
It takes a lot to rattle her.
It takes at least a week to acclimatise to the altitude.
And of course:
It takes two to tango.
Yes, because "take" can mean "require." If three psychologists are standing in line, it's like taking one out of the line to change the light bulb. "It" standing for the task. It's informal, but it is still correct as a sentence.
You appear to have misidentified the subject of the sentence. In questions, word order is often inverted.
The subject of the sentence is the word "it," not "many" or "psychologists." The verb must agree in number with the subject. If you were to answer the question, you would say "It takes five psychologists to screw in a light bulb." Hence the correct word to use is "takes."
The issue is likely that the word "it" does not actually refer to anything. This is a grammatical quirk of English. All sentences must have subjects. The word "it" in this sentence is a non-referential subject. More information may be found in this English Language Usage question.
How many does it take - implies that there may not be a specific, immutable number. How many is needed - implies that there is an actual answer that can be given with certainty.
Because psychologists endeavor to help a person achieve change, understanding, and since this process does not follow a predictable path, you cannot answer, "How many are needed?" It remains for the goal to be reached before you can answer the question.