6 Answers

Read here >http://www.tsa.gov/traveler-information/prohibited-items

Here are a few items 

Sharp Objects

While it's probably a no-brainer that you aren't allowed to bring serious hunting knives on an airplane, the fact of the matter is that all sharp objects are prohibited by the Transportation Security Administration. This includes box cutters (which were instrumental in carrying out the 9/11 attacks), knives and even swords. Scissors and plastic silverware are permitted.

Sporting Goods

There are many objects in the sporting goods department that are not allowed on airplanes in this day and age. Simply put, too many of these objects can double as weaponry. This includes baseball bats, golf clubs and archery bows. These items may be put into checked baggage.

Firearms

While guns have long been banned from airlines, it should be noted that this ban includes not only true firearms, but also replica guns and paintball guns. BB guns are also not permitted. Any type of gunpowder and ammunition is also banned from the airlines.

Oddities

Along with what might seem like the usual suspects are a group of items that one might not expect to be banned from flying. These include gel sole inserts for shoes, liquids or gels in a container larger than 3 ounces and even snow globes. Chemicals and other flammable items such as spray paint or liquid bleach are prohibited as well.

Lists of items that are not allowed on an airplanes are posted at links listed at 
https://www.google.com/search?q=items+not+allowed+on+an+airplane&oq=items+not+allowed+on+an+airplane&aqs=chrome.0.57j60.94&sugexp=chrome,mod=1&sourceid=chrome&ie=UTF-8#hl=en&sclient=psy-ab&q=list+of+items+not+allowed+on+an+airplane&oq=list+of+items+not+allowed+on+an+airplane&gs_l=serp.3..0i30j0i22l2.3267.5885.0.6149.10.9.1.0.0.6.535.2349.0j4j3j1j0j1.9.0.les%3Bcpmergendl..0.0...1.1.pwsQd8hp5XA&pbx=1&fp=1&bpcl=38625945&biw=1366&bih=667&bav=on.2,or.r_gc.r_pw.r_cp.r_qf.&cad=b&sei=tRupUMyKNueS0QXP-YCoDQ. 

guns

knives

bombs

I seem to remember that mercury is banned as it can rot the airframe if it escapes.

http://www.tsa.gov/traveler-information/prohibited-items