Consider Jane, an enthusiastic rambler. Some years ago she very sensibly invested in an expensive, state-of-the-art pair of hiking boots. Come rain or shine she goes out walking and over the years they have served her very well. They are comfortable and for the most part they keep her feet dry. They are a little worn around the edges but over the years they have done the job. 

A couple of years ago Jane impulsively bought a second pair of boots. They have hardly ever been used and she has often wondered whether they were a sensible purchase -that is until a bout of exceptionally severe weather last winter. Three inches of snow fell in a couple of days and for week or so thereafter, the ground was frozen solid and walking even short distances outdoors was very hazardous.

Undeterred by this spate of bad weather after donning her favourite boots Jane ventured out. She quickly realised that they did not have quite enough grip and that her feet were a bit damp. She also realised that were she to wear a pair of ordinary shoes they would be ruined. Remembering the second pair of boots she went back inside, put them on and had no further difficulties. When the weather improved she went back to wearing her favourite boots and in all probability will be able to do so for quite some time to come.