Many common health myths start from honest misunderstandings. However, separating fact from fiction can help you make smarter choices about your health, from the common cold to a nose bleed. Here’s what you need to know.

From your skin’s surface, the veins in your body may appear deep blue or even purple. But that’s not an indication of the color of the blood inside your veins. Your blood is actually red. The blue hue of your veins has more to do with how your eyes absorb and see color than the color of the blood itself.

The level of oxygen in your blood cells determines the brightness of the red color. Blood pumped directly from the heart is oxygen rich and bright red. As the blood circulates the body and oxygen is removed by tissue, the blood grows darker. For that reason, blood returning to the heart and lungs often has a dark red appearance. But it’s never blue.

This myth’s been debunked many times over, yet it remains a persistent health tip. It may be rooted in good intentions—water is vital for the body’s everyday functions. However, your body is a well-tuned machine that will give you many signals before you’re ever dehydrated.

Additionally, the water you take in on any given day comes from many places, not just glasses of water. Fruits and vegetables contain water, as do other beverages like tea, coffee, and juices. Keep in mind that it’s healthier to drink water rather than sugary drinks.

Unlike saturated fat, calories, or sodium, water doesn’t have a formal daily recommended value. Just listen to your body. When you’re thirsty, grab a drink—preferably water. The amount of water you should aim to drink depends on your physical activity, your diet, your weight, your health issues, and even where you live.

Sweat is your body’s natural air conditioning, not its toxin removal system. When you’re hot, your body releases water on the surface of your skin. As that water evaporates, it helps cool your body and prevent overheating.

Sweat is primarily water. A very small portion of the fluid is made up of salt, carbohydrates, proteins, and other minerals from your body.

Sweat contains no toxins. Your kidneys and liver are designed to filter and remove any toxins from your body. This includes heavy metals and drugs. You shouldn’t induce sweating in order to remove toxins from your body. This could be dangerous, even deadly. Your body is designed to remove toxins without your help.

This myth comes from a common misconception about how vaccines work. Vaccines contain weakened or dead strains of a virus. Once injected into your body, the virus stimulates your body’s immune system to fight the particular virus. It isn’t an actual infection, because the weakened or dead virus is easily contained by your immune system. You may still experience some minor symptoms, such as a fever.

When the imitation, or low grade, infection is finished, your body has created a “memory” of the virus. If you come into contact with the live virus in the future, your immune system is equipped to defend against and defeat the virus. Full immunity from a vaccine can take several weeks. If you’re exposed to the virus before that period has passed, you could still get sick. This is because your body hasn’t yet developed immunity. But the vaccine alone doesn’t make you sick.

It’s true that cases of the common cold and the flu peak in winter months. But this doesn’t have much to do with the temperature outside. Germs make you sick, not weather.

In order to get sick, you need to come into contact with germs or viruses. Cold temperatures outside drive people inside. Being around more people provides more opportunities for germs to spread. Dry air from central heating systems also makes it easy for viruses and germs to survive longer, get into your nasal passages, and start an infection.

In other words, cold weather affects the behaviors that can lead to illness, but cold weather doesn’t make you sick.

If this myth were true, many people would be carrying around a bit of extra weight from accidentally-swallowed gum. Fortunately, this myth is false. Your digestive system can’t break down gum the way it can food, but your body will have no problem passing it through your stomach and digestive tract and out via a bowel movement.

If you swallow a large clump of gum or a lot of pieces of gum in a short period of time, the gum could block your digestive tract, but this is unlikely to happen. It’s better to be safe than sorry, so be sure to spit out your gum instead of swallowing it whenever you can.

The first time you shaved your armpits or your face, the hair was likely thin and wispy. Over time, that hair grew back a bit thicker and coarser. That’s the result of hormonal changes that were occurring when you started shaving during puberty, not from the shaving itself.

The truth is that shaving doesn’t make hair come back darker or thicker. If the hair after a shave feels thicker, that’s because shaving actually makes the tip of hair blunt.

If you’re suddenly growing more hair or hair in places you’ve never had hair before, make an appointment to see your doctor. This hair growth could be a sign of an underlying medical issue.

Knuckle cracking can annoy the people around you, but it likely won’t cause arthritis. Around 54 percent of Americans practice this noisy habit, but research shows that those people aren’t more likely to develop arthritis because of it.

Your parents may have begged you to stop eating pizza by claiming it made your teenage acne worse, but they were wrong. Greasy foods have almost no effect on your skin’s appearance. However, some foods may worsen your risk for acne.

Dairy and foods high in carbohydrates may trigger acne, but the same connection can’t be said for greasy foods. If you work in an environment that’s greasy, such as a kitchen with deep fryers, the oils from the fat may stick to your skin and cause blocked hair follicles. This could promote white head growth or make acne worse.

Nosebleeds often start with the blood vessels in the front of the nose, so leaning your head back won’t stop the bleeding. In fact, you may end up swallowing blood, which is very unpleasant.

If you have a nosebleed, sit up straight and lean forward slightly. Allow the blood to flow out while you pinch your nostrils together for at least 10 minutes. This will encourage natural clotting. If you have access to a cold pack, apply one to your nose and cheeks, or the back of your neck. After 10 minutes, release your grip on the nostrils. If the bleeding continues, pinch the nostrils again until the bleeding stops.

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Honest misunderstandings can spread like wildfire. These myths are persistent because many of us grew up hearing the claims and accepted them without question. If you’re ever in doubt about health advice, talk to your doctor. They can provide you with the most up-to-date information and sources.