Often in cooking it is required to have on hand a knob of butter or margarine. 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. However, it never takes that long for someone to come up with an explanation:

A knob of butter is defined as an isosceles right-angled tetrahedron whose adjacent is equal to half the height of the block of butter.

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