Often in cooking it is required to have on hand a knob of butter. This
may be for new potatoes, corn on the cob or many other culinary
delights.

The problem though is this:

How big is a knob?

This question had long been lacking a true mathematical definition.

A 'Knob of Butter' is defined thus:


An isosceles right-angled tetrahedron whose adjacent is equal to half the height of the
block of butter

See diagram

Note: sometimes more than one knob of butter may be required.