One more behaviour of ruby

Today while working with ruby i noticed one more surprising behaviour of ruby.

arg1="Ruby", arg2="On", arg3="Rails" 
puts "#{arg1},#{arg2},#{arg3}"

What should be the results?

Well i was expecting to be printed like “Ruby”,”On”,”Rails” but the result was “RubyonRails”, “On”, “Rails”. To understand this behaviour i did some more experiments in “irb”

puts arg1.class.to_s    
#result  Array 
puts arg2.class.to_s  
#result String 
puts arg3.class.to_s    
#result String

Now the question is, Why arg1 is interpreted as an array? and why not the arg2?