getElementsByClassName() - Returns a node list containing all elements with a specified class
the node list mentioned above is an array. this array consists of only one element, but it is still an array, that's why to get the first element of this array we call array_name[index]. the index is 0 because array's indexes start from 0. my english is not good but i hope this makes sense
xxx: Guess Buddhist riddle: "What is the sound of one hand clapping?"