Suppose that a newspaper published the following in a

boxed corner of the want ads section:
Should handgun control be abolished? You can call the shots if
you participate in our poll tonight. If yes, call 1-900-For-Guns.
If no, call 1-900-Ban-Guns. You will be charged 50¢ for the
first minute.
Why will the results of this poll be biased?

Who reads newspaper classified ads?

What kind of kook will call a toll number to someone they don't know?

So, the poll will be of kooks who enjoy paying to call numbers they don't know.