: According to Stephen Hawking, if two particles directly on the event horizon collide and one of the particles is bounced away, it can escape. : Source: http://library.advanced.org/10148/long13.html

: Anyway, I'm wondering how these two particles can be created out of nothing?

In what we call "empty" space, there is a constant creation of virtual photons that cannot be directly detected. The creation of these virtual photons is explained by the uncertainty principle which states that in any given region both the energy and rate of change in energy cannot be precisely determined simultaneously. This allows for fluctuations which exhibit themselves as short lived pairs of particles/ anti-particles that flicker for a brief instant (10^-43 seconds) and die out in mutual annihilation. Although they cannot be individually detected, their net effect can be detected by placing to metal plates a short distance apart. Only photon pairs with wavelengths less than the separation distance can exist between the plates while outside the plates any wavelengths may occur. This causes a slight inward pressure on the two plates known as the Cassimar effect. It is also a means of getting energy from the vacuum of space (although admittedly impractical with current technology).

Close to the event horizon tidal energy exists in huge quantities. This tidal energy can serve to promote one of the virtual particles to real status at the expense of the other particle which loses energy and becomes a negative energy particle! The negative energy particle is then trapped by the black hole causing a slight loss of mass to the black hole while the positive energy particle is free to escape from the gravity well of the black hole and appears to an observer as if the black hole has emitted a particle. Keep in mind that all this is occurring just outside the event horizon. Anything that cross the event horizon can escape.

: One more question: : It is theorized that if enough energy escapes, the mass of the black hole will become small enough that the black hole will not be able to completely curve the space around it and it will explode with a power greater than millions of atomic bombs. How can a black hole explode?

When the black hole gets small enough that it can no longer maintain an event horizon in space (and we're talking small here, something less than the size of an atom where space can no longer be thought of as continuous), then it is theorized that the singularity will be exposed and what happens is anybody's guess. Most likely an explosion converting whatever remaining mass is left in the singularity into energy. Perhaps the recent discovery of gamma ray bursts are nothing more than primordial black holes exploding!