As you're undoubtedly aware, airport security screening measures and carry-on restrictions for domestic and international commercial flights have increased considerably in recent years. Some rules and regulations get a little complicated, and frequent changes to the guidelines haven't helped.

One common area of confusion is whether you can take liquids with you on the plane, which ones, and in what quantities. It's important to get it right so your family passes smoothly through the security checkpoint and so the TSA screeners don't have to throw away any of your stuff.

Yes. With the exception of substances that are explosive, toxic, corrosive or otherwise hazardous, liquids are not banned from carry-ons. However, the amount you can take on with you is strictly regulated. And these restrictions apply not just to liquids, but also to aerosols, gels, creams, pastes, and all other viscous, oozing, gelatinous, gooey, gloppy, drippy, slushy, sludgy or slimy substances.

So, obviously the liquid rules apply to plenty of items that travelers typically have with them. They cover, for example, toiletries and personal care products like shampoo, body wash, toothpaste, gel deodorant, shaving cream or gel, insect repellent, sunscreen, moisturizer and other lotions, and hand sanitizer; cosmetics like makeup, perfume, hairspray and styling gel; beverages; and food products like peanut butter, jelly, jam, Nutella, condiments and soup.

The TSA came up with a handy little mnemonic device to help airline passengers understand the rules pertaining to liquids. It's known as the 3-1-1 rule.

You might expect the 3 to stand for 3, but it doesn't. Not quite. It actually stands for 3.4. That's the maximum number of ounces of any package of liquid allowed onto a plane in a carry-on bag. If you have a package that holds 3.4 ounces (100 milliliters) or less, you can bring it with you on the plane.

So, a standard 6.2-ounce tube of toothpaste isn't allowed in your carry-on, but a 1-ounce travel toothpaste tube is fine. In addition, it's the package size that matters, not how much you've used; if you have a nearly empty full-size tube of toothpaste, you won't be permitted past security with it.

The first "1" indicates that all your small liquid packages must fit into a 1-quart, clear, plastic zipper-sealable bag. An individual passenger can't carry on more 3.4-ounce packages of liquid than can fit in this bag. While you don't have to put them all into one of these bags, it's a good idea. That way, you know you don't have too much. Also, placing the bag separately onto the X-ray conveyor belt makes your screening faster and easier.

The second "1" refers to the fact that you're allowed one of these bags per passenger. So, if you're a family of four flying, you can take four 1-quart bags total.

Of course they do. The TSA isn't unreasonable, and understands that sometimes people fly with children or medical conditions that call for some wiggle room on the liquid limits. Prescription and over-the-counter medications are exempt from these regulations. So, are nourishing fluids for your kids, like baby formula, breast milk or juice.

Also, if you're boarding a U.S.-bound international flight and taking a connecting flight, you may carry on securely sealed duty-free liquids in packages larger than 3.4 ounces. The liquids must be packed by the retailer in a clear plastic bag, which can show no signs of tampering, and you need the receipt to show that the purchase was made within the last 48 hours.

When it's your turn to get screened, show these exempted items to the TSA agent right away. It helps keep things moving along efficiently, and the liquids may be subjected to additional screening.

Conveniently, most toiletries and personal care products are now sold in travel-size packages allowed in carry-on bags. And most original make-up containers and perfume bottles are of an acceptable size as well. If you need a larger perfume bottle than is allowed for your family's trip, consider that you might be one of those people who wears way too much and bothers everyone around you.

Otherwise, if you need packages of any non-exempted liquids or liquid-like substances that are larger than 3.4 ounces, you have two options. Either buy the item(s) when you arrive at your destination, or pack them in your checked luggage where there are no size restrictions for liquid containers. Just be sure they're tightly sealed.