Bike/Horse Rule
A principle invoked when urging someone to persevere despite initial failure, which fully stated is: "You don't need me to remind you, when things don't go well the first time, try again." So named because it is drawn from two familiar premises: (a) persevering is like riding a bike, you never forget how, and (b) when you fall off a horse, you get right back on and give it another go.
My nephew just failed the bar a second time and was ready to hang it up, but I reminded him of the Bike/Horse Rule and told him to sign up for the thing again, he'll get it.