What is that sticky stuff you use to hang papers on a wall?

The reusable putty-like adhesive used to hang posters on walls was originally sold under the brand name of Blu-Tack. Variations of the product are available for purchase under such names as Sticky Name, Fun-Tak, and Elmer's Poster Tack.

Blu-Tack and its variants are pressure-sensitive adhesives composed of synthetic rubber, polymers, oil and other inorganic fillers. The product was created in 1971 by Bostik employee Austin Carpenter after his wife expressed a desire for a reusable glue. The original product was designed to be white in color like other glues but was changed to blue after concerns were raised that children might mistake it for candy.