Basic Trope: Aliens can live quite comfortably on a planet other than their own, with no problems.
- Straight: The Greyvians come to Earth, and make friends with Alice and Bob. They are able to eat pizza with their new Earth-buddies, and quite enjoy it too. They can also become infected with the same diseases as humans.
- Exaggerated:
- Earth's biosphere is somehow better for the Greyvians than that of their native planet.
- The Greyvians are able to eat literally anything, even that which would be toxic or unpalatable to humans, breathe comfortably anywhere, including the surface of Venus and any disease on any planet can affect them..
- Downplayed:
- The Greyvians can take off any space helmets or gas masks safely, but they either cannot or will not eat pizza.
- Limited subsets of compatible species exist. Greyvians have encountered thousands of sapient alien planets but Earth is the only other one compatible with Greyvians without any other measures.
- The Greyvians can adapt to any other habitable world but it is a lengthy and taxing process which requires cocooning themselves with a sample of diet, proportionate water, and atmosphere conditions and emerging in a form adapted to it.
- Earth food doesn't harm the Greyvians, but it tastes like rotten garbage to them.
- Justified:
- The Greyvians turn out to be descended from humans who were abducted in prehistoric times.
- The lifeforms of Earth and the Greyvians' descend from the same ancient microorganisms. Thus, while outwardly very different, they share a basic biochemistry and can eat each other's foods and catch each other's diseases.
- The Greyvians are cyborgs or genetically modified. A side effect is that the diseases of the planet also affect them.
- The Greyvians are shapeshifters that can take on a human or human-like form that imitates our own biology.
- Inverted:
- The Greyvians take a breath of Earth's air and immediately suffocate.
- Greyvians aren't kidding when they claim a connection to their planet - if they leave it they immediately die regardless of facilities of the spaceship.
- Subverted:
- One of the Greyvians starts choking and gagging on the pizza.
- The Greyvians are served beer, and get two or three times as intoxicated as Alice and Bob do on one beer.
- Pizza has some effect on the Greyvians (such as a narcotic effect, or an aphrodisiac one)
- Double Subverted:
- Because they bit off more than they can chew, or simply does not like the taste, not because pizza is "toxic" in any way.
- Humans are able to survive on the Greyvians' homeworld, but the Greyvians aren't able to survive on Earth.
- They are not used to alcohol, but as they start drinking more beer with Alice and Bob, they develop tolerance. Soon it takes two or three beers to get drunk.
- The effect is temporary and harmless.
- Until a human begins to have the same symptoms
- Parodied: There's a whole intergalactic version of Phantom Gourmet, Guy's Big Bite, or other such shows.
- Zig Zagged:
- The Greyvians are able to breathe air on Earth, but they can't get wet. They can eat pizza, but they find beer to be extremely toxic to them.
- Earth's biosphere is nonhostile to them and capable of suporting their biology... but not for the same reasons as humans. They breathe the Nitrogen in the atmosphere rather than the oxygen, and get sustenance from eating Earth's rocks rather than its organisms.
- Averted:
- Earth's atmosphere is harmful to the Greyvians.
- The Greyvians are not living creatures at all, but robots.
- Enforced: "We need our aliens to be less... alien, so we can teach An Aesop about tolerance."
- Lampshaded: "So, I take it you like pizza. Do they have it on your planet, too?"
- Invoked: The Greyvians arrive just as Alice and Bob are handing the pizza guy money, and since it would be rude to eat in front of them, Alice and Bob offer the Greyvians a slice.
- Exploited: A Mad Scientist tries to find something they are unable to consume safely.
- Defied:
- The Greyvians do not visit Earth or any other planet besides their own.
- The Greyvians wear full spacesuits outside their vehicle, and do not eat or touch anything unfamiliar to them.
- During a Greyvian invasion, Human scientists alter Earths atmosphere to be toxic towards Greyvians but not us.
- Discussed: "I wonder if aliens could eat pizza safely?"
- Conversed: "This show makes no sense. How do those aliens breathe on Earth? Are we to believe that we could breathe on their planet?"
- Played For Laughs: The Greyvians politely (or maybe not so politely) decline the offer of pizza, even though it is not toxic to them.
- Played For Drama: The desire for Earth food combined with its addictive qualities causes the Greyvians to be easily manipulated by those who supply it, even into slavery.
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