I just bought a Mandolin.....WTH?

I've got an 76 key Yamaha keyboard (PSR-GX76), rooster.co.uk%2Fimages%2Fyamaha-psr-gx76-portable-keyboard-x-frame-stand-used-p37928-64672_image.jpg
it's old (20 years at least) but its packed with voices, instruments and beats. Fun to play around with but I know zip about reading music, and that was after 'playing' French Horn for at least 2 months in 4th grade and Mom being a Piano teacher! My oldest sat down on it one rainy Saturday and was playing music from a simple song book by noon! Wife plays piano, middle boy plays guitar pretty good, I can only play "Peter Pumpkin Eater" with one finger.