Some people will boldly plunge up to half their cookie all at once in their drink, other people will moisten an edge at a time.
Dunking is done more in England than it is in Scotland.
Digestive Biscuits are the most reliable biscuits to use for dunking. Ginger Nuts are also good.
Research into all this was actually conducted into 1998 by Len Fisher, a physicist at the University of Bristol.
As simple as cookies are, there is actually a lot of science behind what makes for a good “dunking” cookie. Starch molecules in cookies are attached to each other by sugar. Liquid will swell the starch, making the biscuit soft to eat, but it will also dissolve the sugar holding it all together. If the cookie absorbs moisture too fast, it will disintegrate completely into your cup of tea. A good dunking cookie is one in which the sugar will not absorb moisture too quickly.
Italians dunk biscotti into “vin santo.”
The Romans dunked hard biscuits in wine.