An issue that often comes up in the teaching of introductory physics classes is students’ intuitive preconceptions of how the physical world works — often incorrect ideas about forces, velocities, interactions, etc. There’s a lot of education research on this and how to address it — some of which I’m even reasonably aware of! — but what I decided to write about is something still more primitive.
Last Sunday, I was helping my older son (almost 8) with a science fair project. He loves cars, and decided to see what’s the “best” tilt angle for a ramp that toy cars would roll down. He was rolling out a tape measure to measure distances, unfurling foot after foot along our living room floor, and my younger son (turning four today!) pointed out, “I bet it will keep going forever.” The older one rolled his eyes and pointed out that of course it won’t.
At what age does it become obvious that a few-inch metal box can’t generate an infinite supply of tape? How does this happen? What would a chimpanzee think? (Over a decade ago, I read a book related to the last question called Folk Physics for Apes, http://www.amazon.com/Folk-Physics-Apes-Chimpanzees-Theory/dp/0198572190/ . I remember liking it, but I can’t remember anything of what’s in it now! After some searching, I found the semi-critical Nature review that led me to the book in the first place: http://www.nature.com/nature/journal/v409/n6817/full/409133a0.html)
It’s immensely enjoyable to watch my kids figuring out the world. Sadly, these years will pass…
The Eighteenth Elephant 
I love the idea of an infinite tape measure. So hopeful.
There are a lot of old folk tales with never-ending pots of food, or jugs of milk — maybe we need to make more modern versions with infinite tape-measures! — Raghu