This is in the spirit of the What is a Word™/Phrase™ series started by JLee with a special brand of Phrase™ and Word™ puzzles.

If a word conforms to a special rule, I call it an Eco-Friendly Word™.

Use the examples below to find the rule.

$$\begin{array}{|c|c|}\hline \bbox[yellow]{\textbf{Eco-Friendly Words™}} & \bbox[yellow]{\textbf{Not Eco-Friendly Words™}}\\\hline \text{PAPER} & \text{PLASTIC}\\\hline \text{HYDROELECTRIC} & \text{COAL}\\\hline \text{GAS} & \text{OIL}\\\hline \text{RECYCLE} & \text{TOSS}\\\hline \text{BIODEGRADABLE} & \text{LANDFILL}\\\hline \text{BIODIVERSITY} & \text{TOXIC}\\\hline \text{ENVIRONMENT} & \text{CITY}\\\hline \text{CONSERVATION} & \text{LOSS} \\\hline \text{POLLUTE} & \text{SOIL} \\\hline \text{SLUDGE} & \text{COMPOST} \\\hline \text{EXTINCTION} & \text{VITALITY} \\\hline \end{array}$$

CSV Version:

Eco-Friendly Words™,Not Eco-Friendly Words™

These are not the only examples of Eco-Friendly Words™, many more exist.


Use the title to help you find the solution.

Note: You must get the answer and explanation for the title completely correct. Partial gets upvoted=)


It looked at first as if an Eco-Friendly Word is one that

shares at least one letter with the word GREEN

but as Volatility kindly pointed out in comments when I proposed this

LANDFILL, which contains an N, is explicitly labelled as a non-Eco-Friendly Word

so this couldn't be right. Still, something along those lines seems likely. So, if the lists in the question are correct, then

(within those lists) letters BEGHMRU are found only in EF words, letter F is found only in NEF words, letters ACDILNOPSTVXY are found in both sorts of word, and letters JKQWZ are found in neither. If EF words are exactly those containing at least one letter in a certain set S, then S must contain G (because GAS contains no other letter not also found in NEF words); it must contain E (because despite its length EXTINCTION contains no other letter not also found in NEF words); and if it contains those two then all EF words in the list are covered. So S must include G and E and must not include any of ACDFILNOPSTVXY; it may also include some or all of BHMRU and JKQWZ. So, e.g., we could say that EF words are exactly those sharing a letter with BUGGER or GEM or HUGE or EMERGE or HUGGERMUGGER.

If anything of this sort is right then

perhaps the most plausible is HUGGER, as in "tree-hugger". Unfortunately there are lots of Ts in NEF words, so the answer definitely isn't that an EF word is one that shares letters with TREE-HUGGER.

Of course another possibility is that

an EF word is one that doesn't share any letters with some other word. We might hope the word in question had some anti-environmental significance. Alas, the only letters we could possibly form that word out of are FJKQWZ and I don't think there are any suitable word (or any words at all) formed from just those letters. Fjqk!

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  • $\begingroup$ Wait, really? :/ $\endgroup$ – Deusovi Feb 4 '17 at 22:07
  • $\begingroup$ Looks like it to me. $\endgroup$ – Gareth McCaughan Feb 4 '17 at 22:27
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    $\begingroup$ I thought this before, but "LANDFILL" isn't Eco-Friendly (unless that's an error on the OP's part?). $\endgroup$ – Volatility Feb 4 '17 at 22:32
  • $\begingroup$ Oh! I totally missed that one. Will edit my answer accordingly. $\endgroup$ – Gareth McCaughan Feb 4 '17 at 22:36
  • $\begingroup$ Yes, it was "hugger". I was going to use "green," but I wanted to be a bit tricky $\endgroup$ – suomynonA Feb 5 '17 at 4:56


Eco-friendly words use the letter E
The title might be explained by E+co -> friendly where co can be seen as meaning "part of the group"(coexist, cohabit, cooperate, co-worker, etc...)

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  • $\begingroup$ Not only do they use the letter E, they are also semantically linked to ecological conservation (i.e. are words that would come up when talking about ecological conservation). $\endgroup$ – GrimGrom Feb 2 '17 at 14:32
  • $\begingroup$ @Silenus so does the majority of non eco words though. $\endgroup$ – stack reader Feb 2 '17 at 14:41
  • $\begingroup$ Right, but the property of being an Eco Word may be a conjunctive property: being such that it contains an E and is an ecological conservation word. The Non-Eco words fail the first condition, so don't have the property. $\endgroup$ – GrimGrom Feb 2 '17 at 15:09
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    $\begingroup$ They don't have to have an e, although this is partially correct. I changed one word to show an example $\endgroup$ – suomynonA Feb 2 '17 at 18:21

Is an Eco-Friendly Word™ one that:

contains the letter G or the letter E?

It's related to the title in that

GE (General Electric) has been a big promoter of “green” technology lately.

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  • $\begingroup$ You are closer than the above answer, but this is still only partial. $\endgroup$ – suomynonA Feb 5 '17 at 4:47
  • $\begingroup$ @suomynonA: Are there any EF words that contain neither of the letters I suggested? $\endgroup$ – dan04 Feb 5 '17 at 4:52

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