1.   Do not let the liquid boil.

2.   Make sure the liquid does not boil.

3.   Once the jelling agent has been added, if the liquid boils, it will not set.

4.   The essential point to remember, Ms. Willan writes, is that when poaching marrow that has been extracted from the bone, the liquid must never boil.

5.   When liquid boils, stir in dissolved cornstarch to thicken the sauce.