Acids

An acid is a substance with a pH of less than 7.

seed driving a car that won't go faster than 7 miles per hour

A seed (acid) is driving a car that won't go any faster than 7MpH (acids are less than 7 on the PH scale) because it is so small it can't push the accelerator pedal down any further (so an acid pH is from 1 to 7).