He made a face at the sour note. "String must be out of tune." He recognized the sound as being flat so he moved his left hand to the tuning knob and tightened the string then strummed it with her finger. He listened to the sound carefully and determined that it was in perfect tune. "There it is. All of the other strings are in tune so no need to worry." He then held her left hand again and pressed her finger down on a fret then guided her other hand over the string and strummed it making a C chord. "Once you get the hang of the frets you can then make up your own bass lines with ease."