Linear Regression Models (1)

1) "In regression models, there are two types of variables:
X = independent variable
Y = dependent variable
Y is modeled as random.
X is sometimes modeled as random and sometimes it has fixed value for each observation."

I don't understand the meaning of the last line. When is X random? When is X fixed? Can anyone illustrate each case with a quick example?


note: also under discussion in "Math Help Forum" & "SOS math cyberboard"
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For example, if we have height v.s. age (Y v.s. X), is X fixed or random?

I can't understand how X can possibly be random, can someone please illustrate with an example?
Also, what does it mean for X to be FIXED? If we have five data points, x1,x2...,x5, and NOT all of them have the same value of X (e.g. x1≠x2), is X fixed in this case?

Thank you!
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