Science & TechMental model
Check the units before you trust the number.
Every meaningful physical equation has the same units on both sides: metres stay metres, seconds stay seconds. If your calculation yields 'kilograms per euro' where metres belong, it is wrong, however pretty the number looks. The trick goes further: often the unit balance alone reveals what a formula must look like, such as a spring's period not depending on gravity.
Useful: Carry the units through every back-of-the-envelope calculation. They catch most mistakes before you make them.
Source: en.wikipedia.org