"Die weißen Blumen trinken Wasser."

[Cor_Insuperabile]

I have never seen such flowers which drink water :P

[Lenkvist]

It's a personification, a metaphor that gives human traits to non-human entities. Not everybody might be familiar with the absorption of water through osmosis. The process is made more understandable by comparing it with something that people know: drinking.

[Gcoid]

Then why do you water plants?

[Procrastinans]

Does "trinken" mean "slurp up a liquid with your mouth" or just "absorb liquid"?

[Procrastinans]

Also, why is it "weißen"?

[MoonDust]

It's plural.

[muitosabao]

Plural weak inflexion (preceded by definite article Die), in the nominative.
http://en.wikipedia.org/wiki/German_declension#Attributive_adjectives