Gum.

Back in May, me and my friend visited London at the end of a 3 weeks trip in the UK. On millennium bridge, close to the Tate and the Shakespeare's Globe we noticed a guy painting on the floor... He has spent and still spends hours painting the gums let by chewers on the bridge. You can see them all over the bridge and the artist is very easy to talk to.
Street art, purely and basically.