Fire and Water
This illuminating centerpiece uses two vases -- one placed inside the other -- to create the illusion that a lit candle sits underwater.
Source: Martha Stewart Living, July 2007
Larger glass vase
Smaller glass vase
Place a glass vase inside another that is slightly larger (secure them with floral adhesive), and then pour tinted water in between them.
For added light play, make a series of these centerpieces in different shades of the same hue (a few extra drops of food coloring will yield the desired contrast).