In the first half of the 20th century, the first plastic objects:
The method of manufacturing the bakelite had been discovered in 1906 by Baekeland, hence the name of the polymer.
Leo Hendrik Baekeland 1863-1944
Here is a film of the UFA (German) dating from 1930, which shows the popularity of bakelite:
The mechanism is complicated. Because of its mesomerism, phenol has three nucleophilic sites enabled:
By the inductive effect, formaldehyde has an electrophilic carbon atom :
All the conditions are in place that in these sites there should happen a nucleophilic attack followed by water departure. Hydrogen ions binding on the oxygen atom of formaldehyde promote the reaction.