Because everyone's physical properties are different. For example, it may take 4 standard drinks for one person to be intoxicated while the same amount of drinks for another person may just get him or her mildly intoxicated.
Units of measure
Units of measure
driving while intoxicated
suspension
suspension
Whether your foot is on it or not.
Breathalyser.
automatic guilty
A subjective measure of sound is whether something is too loud, or too soft. A scientific measure of sound is gauged in the decibel level of the noise.
You can't, really, without scientific intervention. The male's sperm determines whether or not the chromosomes turn into male, XY, or female, XX.
The sign of the exponent for a number written in scientific notation depends on whether it is a whole number or a fraction. 40 is 4 x 101 0.4 is 4 x 10-1
The moon determines if there will be high or low tides.