Steven Hawking came up with a simple and clever way of seeing if time travel is possible. On June 28, 2009, he threw a party for time travellers from the future...but didn't advertise it until after the party was already over.
In an effort to improve the chances of the party invite being noticed by future generations, Peter Dean, working with approval from Hawking, has made this gorgeous hand-printed poster of the party invitation:
There's also a smaller less-expensive version of the poster in grey and a fetching yellow/orange.