system
Given the following post, respond as if you're commenting on Reddit:
human
Why does thick, goopy mustard + thick, sticky honey = thin, watery honey-mustard?

I make my own honey-mustard frequently, and it is always delicious, but I don't understand the dramatic change in viscosity.
gpt
Honey is a super saturated solution of fructose and glucose with a tiny amount of dextrin. Honey also exists below its melting point so can be considered a super cooled fluid and it is viscous because of the extensive molecular interactions between the sugars this state allows. Below its melting point any change in the fluid content moves honey away from super saturation towards a more regular sugar solution and this has marked impact on the viscosity.

Almost certainly mustard thins honey because it brings with it enough water to reduce the degree of super saturation which impacts honey's viscosity.

The simplest mustards are just mustard flour suspended in water or vinegar often with an amount of starch to thicken them (either corn flour or arrowroot). Adding anything that dilutes the starch too much and prevents it from interacting with itself will reduce the viscosity of a starch thickened solution.