Why doesn't candle wax burn without a wick?

The wick is the most important part of a candle! The mechanics of a burning candle are actually quite interesting. The wax, in it's solid form, does not burn. When you bring the flame of a match near the wick, the wax directly below starts to melt due to the heat (or in some cases, the actual wick material will start to burn and then heat up the wax below). The melted liquid wax is then drawn up to the flame on the wick through capilary action. As the liquid wax is drawn up into the flame, it is heated until it vaporizes. Once it is in gaseous form, it is ignited by the flame. The combustion of the gaseous wax keeps the flame going and generates enough heat to warm up some more wax. The cycle continues as the candle wax is burned away. Nifty!

Why do our eyes tear when we cry?

Scientists distinguish three kinds of tears, basal, reflex and emotional.

Emotional tears are the ones that occur when we cry. These tears contain high concentrations of hormones that build up when the body withstands emotional stress. The reason people will frequently report feeling better after a well-placed cry is doubtlessly connected to the discharge of these stress-related hormones.

In case you are curious about the other types of tears, basal tears are the secretions responsible for keeping the eye lubricated and flow continously as we blink, and reflext tears are the secretions that occur to flush out irritants (dust, hair, etc.) from our eyes.

Resources:
Tearful Serenity
Cool Quiz

Why Do Smoke Detectors Contain Radioactive Materials?

One of the most common types of smoke detectors is called an ionization detector. This type of smoke detector contains a small quantity of Americium-241, a radioactive element that emits alpha particles. The alpha particles emitted by the americium ionizes the air molecules (oxygen and nitrogen) allowing electric current to flow between a set of metal plates hooked up to a battery. When smoke is present, the ions attach to the smoke particles and are neutralized. This reduces the electric current between the plates which triggers the detector to sound the alarm.

Before you worry, the alpha particles emitted by the americium do not travel very far in air and are not any danger to you. This is as long as you don't DISTURB the americium. Americium can be very dangerous if inhaled. So do not tinker with the smoke detector or try to examine/play with the americium.