It’s 2008. I’m ten years old. I have a henna cone in my hand and I’m not afraid to use it. I squeeze unrecognisable flowers onto the women, and I paint wonky footballs and messy rockets onto the boys.

Sometimes the best inventions and ideas come at us unexpectedly. The Slinky. Post-It notes. Penicillin. The same could be true of art, as the Tumblr accidental aRt proves. The site features ...