I also photographed this Timber Rattlesnake (Crotalus horridus) through thick plastic at the Chattanooga Aquarium. Unfortunately, the barrier was smeared, so the photo is a bit blurry. But, hey, I'm not going to be too picky where rattlesnakes are concerned!
Rattlesnakes are venomous, not poisonous (you can eat rattlesnakes)! Venom is injected through either a bite or a sting; poison is taken into the body by touching a substance or eating it. I teach my students that "Venom" starts with "V". I hold up my index and middle fingers pointing the tips down in a biting motion like snake fangs. To simulate a stinger I make the bottom of the "V" go down. Since the word "poison" starts with a "P" (which is round at the top), think of the tip of a finger and the tip of the tongue as being round; poison must be touched or tasted (eaten).
Note the vertical pupil in the eye and the pit near the nostril, only venomous snakes in North America have these traits. Rattlesnakes are pit vipers, they use the pit as a heat or infrared sensor. They can "see" warm-blooded prey, such as a rat or mouse, in pitch dark. Rattlesnakes grow a new button segment on their rattle every time they shed their skin. They may shed more than once a year and they can lose buttons, so the number of buttons on the rattle is not a reliable indicator of how old the snake is. To some people, rattlesnakes seem to have an "evil" look because of the ridge over their eyes and their unblinking stare! The ridge protects their eyes from glare and they don't have eyelids to blink. All snakes shed the covering over their eyes when they shed their skin. It is often easy to tell when a snake is getting ready to shed because the eyes will have a milky look to them as the skin loosens.