Hydrangea, originally uploaded by AMK.

From Wikipedia:

In most species the flowers are white, but in some species (notably H. macrophylla), can be blue, red, pink, or purple. In these species the exact colour often depends on the pH of the soil; acidic soils produce blue flowers, neutral soils produce very pale cream petals, and alkaline soils results in pink or purple. Hydrangeas are one of very few plants that accumulate aluminium. Aluminium is released from acidic soils, and in some species, forms complexes in the hydrangea flower giving them their blue colour.

Although this is technically my hydrangea plant, it lives in my mom's backyard until I buy a house with a yard. Last weekend, my mom fertilized the plants, and within a day, its flowers had morphed from the purplish color in the above photo to a much pinker color (unfortunately, I don't have a photo of that). Apparently, MiracleGro has an alkaline pH.

I find this aspect of hydrangeas fascinating. I always knew the hydrangea's flower color came from the pH level of the soil, but I had always assumed that it was determined when the flower was forming.