To pass this week's challenge, you have to choose to be cold.
With all the conveniences of modern living, it's relatively trivial to get and stay warm. The ease with which we can attain this level of comfort, however, doesn't help make us stronger or teach us to appreciate how good we've got it.
Choose to endure some discomfort as a way to get stronger and to remind yourself that things could always be worse. It also will save on your heating bill, which is nice.