They tell you that cutting your own firewood warms you twice. It's a lie, but it's a witty lie. It's one of those lies that has truth in its pocket; it's a lie because it's more than true, not less. Therefore, in reflecting on its deception, you can not only avoid error, you can also learn.
As anybody who has done it knows too well, cutting your own wood warms you not twice, but three, four, ten, twenty times. The implication that you work up a mild and pleasing heat in procuring a year's worth of fuel and then forget about firewood until you kindle a blaze to cheer a cold winter night is fanciful. The woodcutter each year pits himself against a level of mass, weight, and inertia that is truly gigantic. Indeed, assuming that you own a chainsaw, cutting your wood is the least of the job. The real work of getting your fuel is in moving the stuff from one place to another. You will lift and shift every stick. And again. And again. And then again. The warming you get from combustion of all this matter comes as an anticlimax.
Therefore, when an advocate of antique ways tries to float this warms-you-twice canard by you, reflect. Not all lies are untrue. Here, the lie isn't in what he's telling you, but in what he's not. If you fall from a great height into a vat of vegetable soup, you'll get a carrot in your ear. But you'll get more than that.