I’m thankful for you, reading this article. But I’m also thankful for turkey and potatoes and pecan pie. And in the spirit of Thanksgiving dinner, I’d like to serve you with a smorgasbord today. The appetizer comes from the engineering world. The main course brings in investing. And for dessert, I added a quick calculator to consider the risk of COVID at your Thanksgiving dinner.
Low and Slow
I’m a mechanical engineer. In the engineering sub-field of heat transfer, there’s an important quantity called the Biot number. The Biot (bee-yo) number compares the way heat enters a body at its surface against the way that heat travels through the body.
That might not make sense to you. That’s why the Biot number needs to be explained using food!
Why do we cook pizzas at 900ºF for 3 minutes? Great question, especially when compared against cooking turkeys at 350ºF for multiple hours.
Pizza has a small Biot number. It has a large surface area compared to its volume—it’s very thin. Any energy added to the pizza at its surface will quickly propagate to the center of the pie.
But turkey has a large Biot number. It’s roughly spherical, so its ratio of volume to surface area is vastly larger than a pizza’s. It takes time for energy added at the surface of the turkey to propagate to the center of the turkey.
And then there’s the matter of mass. This is separate from the Biot number, but equally important. Cooking a 20-pound turkey will take longer than cooking a 1-pound pizza. That’s easily understood. Heavy stuff takes longer to warm up.
Potatoes and Pumpkin Bread
Why do I have to bake pumpkin bread at 325ºF for an hour? Why can’t I bake it for 450ºF for 40 minutes? Or in a pizza oven, at 900ºF for a few minutes?
I don’t recommend it, but it’s an experiment you could conduct yourself. You’d find that you’d overload the exterior of the loaf with heat before giving that heat enough time to propagate to the center of the loaf. The outside burns. The inside remains raw. And everyone’s sad at the lack of pumpkin bread.
The more cubic or round or dense a food is, the more low-and-slow the cooking or baking will be. This applies to loaves of bread, cakes and pies, or dense cuts of meat. A meat smoker might run at 225ºF all day.
If a food is flat or thin or narrow, it can probably be cooked high and fast. Pizzas, bacon, stir fries all apply. Lots of surface area and lightweight.
But what about mashed potatoes? We only boil potatoes at 212ºF degrees for 15 minutes. That’s way colder and shorter than a turkey or pie. And potatoes are reasonably dense. What gives?
The answer is that water transfers heat more effectively than air. That’s why 60ºF air feels temperate to your skin, but 60ºF degree water is frigid. That’s why you can stick you bare hand in a 400ºF oven (for a few seconds), but sticking your hand in boiling water (212ºF) will scald you. Water moves heat better than air.
And moving or flowing fluid transfers heat better than stagnant fluid. This is why cold winter air has a “wind chill” factor—the blowing cold air removes more heat from your skin that stagnant cold air. And those Thanksgiving potatoes are surrounded by boiling and roiling water. They cook quickly.
Invest Like a Turkey
Enough engineering. Let’s bring it back to money.
You can approach investing like baking a pizza. Or you can invest like you would cook a turkey. I recommend the turkey version.
You can (try to) pick stocks that will double overnight. Or you could explore exotic asset classes with promises of “going to the moon.” You can even borrow money—or leverage—to further extend your investments. This is investing like a pizzamaker. It’ll be hot and fast and potentially over in five minutes.
But sadly, historical context provides ample data suggesting that pizza investing is not effective. Hand-picking stocks has more risk than reward. Short-term flips are closer to gambling than to investing.
That’s why you should invest like a turkey. Low and slow and long-term. Check on your progress occasionally. Adjust your timeline if needed. A half-cooked turkey does not resemble your final product, just like a half-funded portfolio can’t support your retirement. But mostly, stay on plan and trust the process. Plan for the long-term and let time take care of the rest.
Use last week’s retirement calculator to plan for the long-term…starting with your savings goal for 2021.
A Plate Full of Stuffing
And speaking of Thanksgiving, ensure that your investing portfolio resembles a Thanksgiving plate: diverse and well-balanced.
Could you imagine eating 1500 calories worth of gravy? Well, maybe. But it would be accompanied by plenty of turkey, stuffing, cranberry sauce and potatoes, too. You can even fit in a slice of something exotic, like pecan pie.
Similarly, a well-balanced investment portfolio reduces your risk from being over-exposed to any single asset type. I described my personal choices in my “How I Invest” article. But there are many ways to skin a turkey, and many ways to diversify a portfolio.
Will Your Turkey Get COVID?
Everyone seems to be all huffy about gathering for Thanksgiving. So-called “experts” are saying the holiday will act as a super-spreading event for COVID. First, Starbucks cancelled Christmas. And now China is cancelling Thanksgiving? What’s up with that?!
Don’t be an ignoramus. For most of the United States, a gathering of 10 or more people has a higher than 50% chance to contain at least person who is positive for COVID. Re-read that sentence.
If you’re going to gather for Thanksgiving, it’s helpful to understand the risk involved. For some, the risk is small and reasonable. For others, the probability of COVID being at your gathering will easily surpass a coin flip.
The following calculator is a simple, first-order estimate. It provides an example of how probabilities work. There’s more explanation after the calculator.
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I’m not an epidemiologist or virologist. Please take this math at face value. If an area has a positive infection rate P, then then odds of a person being negative is 1-P. The odds that all N people at your gathering are negative is (1-P)^N. Therefore, the odds of at least one positive case at your Thanksgiving gathering is 1-(1-P)^N.
I recommend looking up your area’s positive case rate here—COVID ActNow. Now, a large positive test rate is just as indicative of insufficient testing as it is of high infection rates. If you only have enough test supplies to test the sickest people, then you’re likely to have a higher rate of positive infections. More reading here from a guy named Johns Hopkins.
So feel free to play around with the infection rate. The true infection rate of an area is likely lower than what’s reported on COVID ActNow.
Keep Grandma healthy!
Thanks a ton for reading the Best Interest. I try to stuff this blog full of fun and helpful information, and having wonderful readers is the gravy on top.
I wish you a happy and healthy Thanksgiving. And don’t burn the pumpkin bread!
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