When photons pass through things like glass, their paths are distorted. If the new paths concentrate into a given area, we see that part as brighter. If we do not need physical perfection, we can fake the effect.
Here we can see the plain scene where we want to simulate the effect. You can get the file of this Fake Caustics of a Glass Ball to follow the exercise.
It is rather simple, we just need to add a light Caustics, a Layer only Spot lamp with a Clouds texture. We place it inside the ball, pointing in the same direction as the light that causes the shadow, Default spot. The Caustics light and the ground plane share layer 2 to avoid side effects. Simple.
Here we see the resulting image. For more complex objects like bottles, we should do two things, first get some reference images to know how it looks in reality, second paint a texture that gives us a similar shape (we can leave the noise part out and add it in Blender, for example to animate it slightly).
For water cases, like pools or the sea, you should get some Caustic Textures and place the light above all the objects.