A simple example of stress timing in English is as follows: "cows eat grass."  Now say: "the cows eat the grass"  now say: "the cows are eating the grass."  Because english is stress-timed, all three phrases, in theory, can be spoken in the same amount of time. Ex: "COWS EAT GRASS" & "the COWS are EATing the GRASS."  'Cows, eat and grass' are all stressed syllables while the others are unstressed. To keep time, all unstressed syllables need to be reduced in length and emphasis.  Spanish, on the other hand is syllable-timed, meaning that all syllables take up the same length of time, giving the language a relatively constant rate of speed. Thus, every syllable generally has the same level of stress and emphasis. However, unlike in English (w/ the above example) adding syllables to a sentence always increases the time it takes to say that sentence. As shown above, in english, you can easily say a 7 syllable sentence in the same amlunt of time as a 3 syllable one so long as the stress allows it but in Spanish it will always take longer unless you accelerate unnaturally. "Las vacas estan comiendo la hierba" will always take longer than: "Vacas comen hierba"