Amusing puns and double-meanings

The pun (a humourous device exploiting two words or expressions sounding the same with two different meanings, usually with two different spellings) is one of the great wonders of the English language.
100% reliable contraception is inconceivable.
Serious campers are intense.
Time flies like an arrow; fruit flies like a banana.
Sports people can avoid the pain of defeat by wearing comfortable shoes.
Nut screws washers and bolts. (Headline following a laundrette sex crime)
Poetry written upside-down is inverse; poetry of very few lines is universal.
A girl who screamed and shouted for a pony got a little hoarse.
The carpenter's heavy tools were uncomfortable so he got a little sore.
Nuns generally wear plain colours because old habits never dye.
The days of the pocket diary are numbered.
Lions eat their prey fresh and roar.
Old bikes should be retired.
Geometry holds clues for the meaning of life; look and you will see the sines.
You can't beat a pickled egg.
If a leopard could cook would he ever change his pots?
See one melée of unruly people and you've seen a maul.
Do hungry time-travellers ever go back four seconds?