This week, the fifth offering in the Die Hard action franchise hits movie theaters nationwide. The series has always tried to avoid simply calling its sequels Die Hard followed by a number, starting by giving Die Hard 2 the subtitle Die Harder, and dubbing episode three Die Hard With a Vengeance. But the fourth installment, Live Free or Die Hard, and the soon-to-be-released A Good Day to Die Hard both rely on a particular kind of wordplay to grab our attention. Start with a well-known expression — say, New Hampshire's motto "Live free or die," or the Klingon-popularized "A good day to die." Overlap the matching part of another expression, "die hard" — and voila! The result is a phrase that makes you play mental tug-of-war as your brain associates the overlapping part first with one expression, then with the other. It's a bit like watching a Necker cube.

This kind of linguistic overlapping shows up in a variety of places in English, from small-scale omissions inside individual words to entire clauses shared between sentences.

Within a word, omitting one sound or syllable that occurs twice in a row is known as haplology. (Witty linguists sometimes call it haplogy — see what they did there?) If you remember the reproduction unit of your high-school biology class (and who doesn't?), you might recognize the root haplo­ from the word haploid. That's the word that describes sperm or egg cells, which have only one copy of each chromosome instead of two. Examples of haplology in English words include libary (instead of library), humbly (from humble-ly), and "Kemmer" — the local pronunciation of Kemmerer, Wyo., where I once had a summer job digging up fossil fish.

When words collide to form "portmanteau words," typically each word loses some of its sounds. This is what happens with portmanteaus such as liger and spork. Sometimes, though, an identical string of sounds at the end of one word and the beginning of the other allows for a blend in which neither word has to give up anything. In a portmanteau such as bromance, everything is kept intact. Like an electron shared between two covalently bonded atoms, the ro belongs to both bro and romance. The same thing happens in guesstimate and netiquette.

Or how about sweet tooth fairy? That's the term invented by one Graham Hidderley-Burgess to describe a combination of two compound words, one of form A B, and one of form B C, with the B part pronounced only once. His canonical example gives the construction its name. On his website, you can find emotional baggage carousel, victory lap dance, and periodic table manners. And in Lewis Carroll's Through the Looking-Glass, you can find rocking-horse-fly and bread-and-butter-fly. In addition to these playful creations, boring and unremarkable examples do exist. For example, restaurants serving milkshakes made with real milk instead of pumped out of a hose usually call them real milkshakes. A guitar solo performed on an acoustic guitar could be called an acoustic-guitar guitar solo, but realistically, acoustic guitar solo is what you'd say.

Gertrude Stein's most famous quotation overlaps the end and the beginning of two entire sentences — or more precisely, the same sentence twice — to produce the ungrammatical yet memorable A rose is a rose is a rose. In ordinary conversation, sentences like That's all you need to do is to think positively happen all the time, running together That's all you need to do and All you need to do is think positively.

Do you have examples of sounds, syllables, or words that are so nice you don't say them twice? Leave a comment!