For all the sign-deciphering I did during my months in Japan earlier this year, I'm surprised how long it took me to notice "No littering" signs, which are often rendered in a katakana + kanji combination: ポイ捨て禁止 poi-sute kinshi 'casual-discard forbidden'. The verb 捨てる suteru means 'to throw away, discard' as in the famous legend of Obasuteyama, a mountain where old women were left to die after outliving their usefulness. (I'm not sure if the men were left on a separate Ojisuteyama.) And ぽいと poi-to means 'casually throwing or discarding something' (according to my electronic Super Daijirin).
The photo above shows a set of four bilingual anti-smoking signs that can be found in many train stations in Japan. The English version of the sign on the bottom right says, Posters saying "Don't litter with cigarette butts" are like children scolding adults with paintbrushes. It must be a Zen koan of an advanced type that exceeds my level of enlightenment, because I don't quite get the point.