Most pet owners know about some obvious things that pets need to be shielded from, like cleaning supplies, home chemicals and even bottles of medicine.

But there are some common foods in every home that could prove surprisingly lethal to our four-legged friends.

Most pet owners already know that it's a bad idea to give your dog or cat any chocolate, but few people know precisely why.

"There are two different chemicals that are found in chocolate that cause problems for dogs -- theobromine and caffeine," said Dr. Heather Bennett of the Denver Dumb Friends League, the local humane society. "Interestingly enough, milk chocolate has the lowest levels of these ... in dark chocolate, it's increased, and baker's unsweetened chocolate is going to be your biggest problem."

And if you come home to coffee grounds scattered around your kitchen, that may not be a good sign.

"Coffee is very bad if they get into it. They don't tend to eat as much of it because it doesn't taste as good as like chocolate cake, but the caffeine levels are much, much higher," Bennett said.

And don't share your burger if it came with onions.

"Onions cause a different set of problems. They cause a hemolytic anemia, which means the body begins destroying its own red blood cells," Bennett said.

And onions aren't always as obvious in food as you would think.

"Baby food very often has onion powder as a way of adding flavor and can cause this anemia in cats as well," Bennett said.

If you think fresh fruit could never be harmful, you're wrong.

"What some veterinary toxicologists have been finding in the last few years ... is that grapes and raisins can both cause acute kidney failure in the animals, meaning that it comes on very quickly," Bennett said.

Experts say it isn't enough to have a cap firmly on prescription or vitamin bottles. A determined dog can chew right through them and hurt himself on the shards as well as the contents.

Protect your pets as you would small children. If it's dangerous, keep it inside a cabinet with closed doors and away from areas where you keep your pet. If you have any questions about something your dog may have eaten, call (888) 426-4435 to talk to poison control experts for pets.

Contact the Animal Poison Control Center for tips on how to poison-proof your home and what to do for a poisoned animal.