Still, after he goes off to bed, she dismisses his insistence to add oak to the fire and adds birch instead. The memories come flooding back and for a brief time, she is young again. And the shadows they saw nothing but a young girl on her honeymoon.
The last line brings everything into focus: She thought of heat. She thought of time. She called it an even trade.
Although the wife misses the way they once were, she also realizes that what she has now is lasting, that it is important, and that her husband's love for her will keep her warm through the cold winter nights.