Wheatpaste

From Wikipedia:

Wheatpaste is a liquid composed of equal parts flour and water, usually made for the purpose of adhering paper posters to walls. Closely resembling wallpaper paste, it is made by mixing roughly equal portions of flour and water (some argue using more water or more flour), and simmering over low heat until it thickens. Allow it to cool to room temperature and then bottle in an easy to carry plastic container.

A similar flour and water formula is taught in elementary school (minus the low heat simmer) as an easy to make substitute for ready made adhesive. A typical application is in constructing streamers of paper rings made from colored construction paper.

Activists and various subculture proponents (such as hip-hop, punk, and anarchist) often use it to hang-up propaganda and artwork in urban areas—usually during the dead of night due to its illegality in many cities. It is applied to the backside of paper then placed on flat surfaces, particularly concrete and metal as it doesn't adhere well to wood or plastic. Cheap rough paper, such as the sort newspapers are printed on, works well, as it can be briefly dipped in the mixture to saturate the fibres. Due to danger of being apprehended wheatpasters frequently work in teams or affinity groups.

Wheatpaste is also known as Marxist glue, probably because of the small far-left organisations that use it.