Poise

hold it flat on your palm. stand on one foot. then shut your eyes.

a heron on one leg in a woodblock marsh sways when you do; three stands, and your own eyes-shut sway ratio

the phone is a level: it reads its own tilt about 60 times a second, and the card measures how much that tilt wobbles about its slow mean while you stand. With your eyes shut you can still stand, but your body loses one of the three senses it balances by (sight, the inner ear, and the feel of your feet and joints), so it corrects later and you sway more. Doctors call the comparison Romberg's test. The number is yours, from this stand: it moves from try to try, and it includes your hand as well as your body.

Concepts: balance, proprioception.

An interactive you play with one thumb, on Wundr.