Close? ... or have I been messing up from the get go?

< ^ > P N >|^_>< --- | ~ ...Help

While doing some reading this morning on nokogiri, I noticed that they used this opening a file:  

f = File.open("blossom.xml")
doc = Nokogiri::XML(f)
f.close

when I've read files, I've never closed them! I've always been under the assumption that reading a file closed the I/O stream automatically when completed. Have I been under the wrong assumption from the get go? 

Wayne