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.