I've posted before about the way in which doing math by slide rule forces one to estimate the answer before “reading” it from the slide rule. You can't get an answer from the slide rule without actually understanding the math in the problem you're trying to solve. Calculators eliminate that requirement, to my regret – and, I think, to the detriment of our young folks learning how to apply math to the real world.
Here's a calculator that changes that, in an interesting way: it won't give you the answer until you've supplied an acceptable estimate. That is exactly how it works with a slide rule. As I read the description of the QAMA calculators, it occurred to me that there's an even simpler way to make a calculator that “behaves” like a slide rule: simply disable the decimal point display! Such a calculator, if you were to (say) divide 1 by 7, might display 142857143 – and it would be up to you to figure out where the decimal point belonged. When I divide 1 by 7 on the slide rule at my desk, I read off the answer as 143 – the same answer as the decimal-less calculator, but to less precision. In both cases, I'd have to estimate the answer in my head to figure out that the decimal place was before the first “1” (the correct answer is 0.143, to three decimal places).
I don't suppose I'd actually be able to talk people into disabling the decimal display :-)