Punny thing...

I've had a long-standing peeve about people not knowing the difference between a pun and a double meaning. Admittedly, the difference is subtle and the humour-ally challenged can argue endlessly, defending their point (assuming they have one). I have been at the receiving end of a few of these arguments and have been made to understand the meaning of the Hindi phrase bhains ke saamne been bajana. The difference, to me, is rather simple. A thing of pun is a joy forever whereas a double entendre is somewhat like a trinket; cheap laffs which rarely have lasting humour value. Finally, puns are not for everyone. No problem with enjoying a sidey joke - I do too, sometimes. Just please don't call it a pun, or say 'pun (un)intended' when it's just a double meaning.
Illustrations are probably in order. These are just off the top of my head, so they may not be perfect. But they may suffice to illustrate the point.

Girl : Do you have a 'pen'? Hee hee...
Boy : Yeah, I have a 'big one'! Hee hee...

This is a simple double entendre, with no pretensions to punning. Now, if the boy making a penis-reference joke were to be called Dick or Peter, that would be somewhat funnier and may even qualify for a pun. But not decidedly so.

Punning is very much a grey area. (pat, pat)

Now, something like " A 'pen' in hand is NOT worth its two deputies in the bush" is funny on several levels (I can hear a few groans in the house and a few cries of 'gutter!gutter!'). I would rather not go into how the above statement is funny, gross, and somewhat clever (in the bargain) all at the same time. I could go on about the pen's ink and inkpots and the like, but I fear bodily harm from certain quarters (those people yelling 'gutter!gutter!' in particular).
The point is that, even though the above stuff is funny on different levels depending on interpretation, it is STILL not punning. It remains on the level of double entendres. Clever ones, but d.es nevertheless.

The humour in a pun, however, is not so easy to explain. Even if explained, it just may not seem funny. Just like the eye of the beholder, the humour in a pun is particular to the 'laugher' (?). So I won't bother explaining the humour in the puns below.

The title of the 1992 Robert Zemeckis film starring Meryl Streep, Goldie Hawn, Isabella Rossellini and Bruce Willis - "Death Becomes Her".

Mercutio, Romeo's best friend, is fatally wounded by Tybalt. On his deathbed, a smiling Mercutio's last words to Romeo are "Ask of me tomorrow, and you shall find me a grave man". Ah, the brilliant bard!

I rest my case.