If there are aliens wandering around, it's surprising that they seem to have no trouble breathing our corrosive alien atmosphere (assuming they need to breathe), coping with our gravity (those slender little ones would probably be on their hands and knees), contending with our lethal microbes, which even we can't always overcome, and if they had the misfortune to meet an average American there's a fair chance they'd be shot. If we went to their planet, how would we fare just stepping off our spaceship and mingling inconspicuously with the locals. Not too well would be my guess.