Once in a while it really hits people that they don’t have to experience the world in the way they have been told to. Alan Keightley
In the aftermath of the World Trade Center and Pentagon attacks, the list of items that may not be brought into aircraft cabins has grown considerably longer, and the actions of security personnel more zealous. Although the Transportation Security Administration (part of the U.S. Department of Transport) publishes an official list of both prohibited and permitted items, its existence doesn't necessary predict what will happen at the airport, where the personal views of random security personnel may have a greater impact on your travel experience. This is even more the case in developing countries, where the rules sometimes seem to be made up on the spot. There's little you can do about any of this, other than be polite: a pleasant smile, a calm attitude, and a little deference (whether justified or not) will always serve you better than grumpiness and belligerence.
Nonetheless, you should familiarize yourself with the official position, to ensure that you are not inadvertently attempting to board with something untoward. It's also advisable to revisit the list occasionally, as it changes from time to time (nail files and small tools, for example, were not always permitted). Finally, appreciate that the above list is only definitive for the United States; other countries have their own rules, which are not necessarily the same. England, for example, forbids scissors (even the blunt-tipped variety), corkscrews, and tools, all of which are allowed in the US.
There are also constraints on the carrying of liquids, gels, and pastes into aircraft cabins. U.S. rules pertaining to this can be found at the TSA link above; European regulations are similar. Travelling with liquids, which are heavy, bulky, and leakage-prone, is best avoided (or at least limited) in any event.