How to insert an automated ‘read more’

Hello,

In addition to the classical ‘more tag’ in the wordpress editor, some themes cut off a post after a specific number of words and insert the tag automatically, even if it is in the middle of the sentence. Actually, this is exactly what I am looking for, but the themes I like to not have this feature.

How and where can I insert such a feature in my current theme? Is there a plugin I can use, or a code I can insert?

Thanks,
Aleks