What's the easiest way to adjoin two fishing lines? The blood knot. And no, you're not going to cut yourself, that's not why it's called the "blood" knot. It's also referred to as the Barrel knot, and is usually used for monofilament nylon lines. Wade Bourne of MyOutdoorTV will show you how to tie the blood knot.
A blood knot (Barrel knot) is most usefully employed for joining sections of monofilament nylon line while maintaining a high portion of the line's inherent strength.
Other knots used for this purpose can cause a substantial loss of strength. In fly fishing, this serves to build a leader of gradually decreasing diameter with an easy cast fly line attached at the large diameter end and the fly or hook at the small diameter end. The principal drawback to the blood knot is the dexterity required to tie it.