Why aren't there eclipses every month?

If the sun, Earth and moon are lined up, shouldn't we get a lunar and solar eclipse every month? Clearly, we don't, but why not?

Solar eclipse over the USA

Mark your calendar.  On Sunday, May 20th, the sun is going to turn into a ring of fire.  It's an annular solar eclipse--the first one in the USA in almost 18 years.

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