This was one of the hardest things I've ever had to pipe. I love Celtic knots and have always wanted to recreate one with icing. (Technically, this is a series of hypotrochoids, not a Celtic knot.) The challenge with this was to make it look like each hypotrochoid was one continuous loop. In reality, each loop has numerous starts and stops, all of which start and end where a loop of another colour crosses over it. This was not too hard to accomplish at the outer portion. The challenge was the inner part where everything is compact and loops are three and four layers high. I first had to analyze on paper where starts and stops should occur. I then piped the pattern a couple of times before I got it right. It's not perfect but I did it and I can honestly say that once is good enough for me!