The last knot on the tzitzit on my talit keep coming loose. I pull the string to tighten the knot, and three days later it starts to unravel again. Any ideas on how I can make the knots stay tight and not loosen?
If your double knots are frequently slipping open, they might be granny knots rather than reef knots. That is, if both layers of the double-knot are tied with the same side on top, the knot is much less secure than if one layer has left on top, and the other has right on top. Untying the second layer, switching which set of strings goes over which, and retying may solve your problem.