Each airline has its own policies regarding the number, nature and size of carry-on items. The TSA has some rules about carry-on items that apply to all flights, however, including various prohibited weapons and restrictions on liquids.
The TSA prohibits all types of firearms, sharp objects and explosives from carry-on luggage. The lone exceptions are for a limited amount of the small compressed gas cartridges used in personal flotation devices and for ice skates. Blunt self-defense items and sprays are also prohibited.
Foods that are liquid or gelatinous in nature are generally restricted to less than 3.4 ounces or 100 milliliters. This includes yogurt, salsa, jelly, gravy and various creamy dips and spreads.
The "3.4 ounce or 100 milliliter" rule also applies to general liquids, aerosols, gels, creams and pastes. The amount of liquid can be increased if passengers are traveling internationally to the United States with a connecting flight, the liquids are in transparent containers or they are bought at a duty-free shop and packed in a tamper-evident bag.
Tools are generally prohibited from carry-on except for screwdrivers, wrenches or pliers that are less than 7 inches in length.
Each individual airline usually lists its carry-on policies on its website.