Unsolicited poetry is not everyone's bag (as some Londoners' ongoing disgruntlement at those TfL adverts will attest), but even the staunchest verse-hater has to admit that secret writing is cool. Remember as a kid, when you used to write hidden messages to your weird little friends using lemon juice? Well, artists in Boston have done pretty much that, but using a biodegradable, water-repellent spray to decorate the streets with lines of poetry. You can't see anything until it rains, when the water darkens the surrounding concrete and makes the hidden words emerge. The public art project is the brainchild of the City of Boston in collaboration with non-profit organisation Mass Poetry, which got artists to spray the pavements with sweet rhymes from a selection of Bostonian poets. Now that's a word on the street.