What Are the Duties of Necessity and Mercy?

A work of necessity is something that perhaps unexpectedly comes upon us through providence. If you have a flat tire on your way to church, you need to change it. But don't go home in the afternoon and rotate your tires. What about preparing for and cleaning up after meals? Now husbands and wives could talk together about ways to make that perhaps less. The idea is that it's lawful to do good on the Sabbath.