The Missing Link: She's speechless.
B.O.B.: She"? It's a boy; look at his boobies!
The Missing Link: We need to have a talk.
Dr. Cockroach, Ph.D.: I'm not a quack. I'm a mad scientist.
B.O.B.: I haven't been outside in 50 years. It feels great!
The Missing Link: It feels hotter than I remember. Has the Earth slowly been getting warmer? It would be great to know that, a very convenient truth.
Gallaxhar: Humans of Earth, my quest has lead me to your planet. Give it to me now! You should, in no way, take any of this personally. It's just business. Gallaxhar out.
News reporter: Once again, a UFO has landed in America, the only country UFOs ever seem to land in.
The President of the United States: Boys, set the terror level at code brown, 'cause I need to change my pants.
The President of the United States: Somebody think of something, and think of it fast!
Chosen answer: Well, it's a fiction movie; some suspension of disbelief is always needed. There could be a number of explanations: for instance, lifeforms such as insects on the grass, bacteria inside them or even grass itself, being unable to absorb quantonium. It might affect only sentient beings. Or Susan might even be the first being to get in contact with the substance - this would be possible depending on the shape of the meteorite container, the exact means by which quantonium touches her body - which isn't shown... but, first and foremost, this happens because its not happening would leave the filmmakers with a plot issue to be dealt with - so, other beings don't grow simply because they don't.