Brush up on the blood knot, the newest Knot to Know.
Commonly used for attaching two fishing lines together, the blood knot is arguably one of the strongest and most efficient fishing knots there is. Joining monofilament lines of two different lengths, diameters, and breaking strengths. It's great for creating the tapered leader that is essential to fly fishing.
It's worth noting that you'll want to snip off the tag ends of the line if you're using this knot to connect two fishing lines. And don't forget to moisten the knot before tightening!