Not all at once! Get some Tapers, and don't stretch it more than a size every 2 weeks, to avoid tearing and give it a little time to heal.
Simply remove your jewelry and leave the piercing empty. There is no guarantee that it will fully close, but as long as you are at a "regular" (18-20 gauge, standard post type earring size) or smaller gauge (4-16 gauge) it should shrink enough to go unnoticed.
If your child is gauging their ears it will heal back if they do not go past a 0'0 gauge.
it is not recommended because the 14 gauge wire is not capable of carrying the same amount of load as the 12 gauge wire which could result in a fire.
252 ÷ 18 = 14
Putting a 14 gauge barbell through a piercing the size that it was pierced at, around a 20-18 gauge, is not recommended because you will be skipping sizes. But if you can put it through with no pain or blood then your'e good to go. You cannot put it straight through without a prior piercing. If it has been pierced and completely healed and already stretched to 16 gauge, yes.
126 (9 x 14, 7 x 18)
First, make sure your ears are completely healed. You can cause a lot of damage if they're not. Second, most ear piercings are either 18 or 20 gauge [20 gauge is rarer] so a 14 or 16 gauge is usually a good place to start although it is recommended to go in order [18, 16, 14, 12, 10...etc.] instead of skipping sizes [18 to 14] Skipping smaller gauges is less dangerous than skipping larger gauges and can be very painful later on [6 to 2] and near impossible to do safely.
14.
20
What is more common on the car is kind of a hard question to answer. Most of the exterior of the car is 20 gauge. (IE: Quarters, Fenders, Hood, Trunk lid.) Roof is 18 gauge. Radiator support: 18 gauge. Inner Fender aprons: 20 gauge. Firewall and front and rear floor pans: 18 gauge. Rear Channel and trunk floors: 20 gauge Most of the Unibodies frame is 16 guage. What I would use is a lot easier of a question: I'd go with 18 gauge for most all my panel repair. (Exterior and Floor pans, etc.) And go with 15-16 gauge for the actual unibodies frame repair.
The Least Common Multiple of 14 and 18 is 126.
2 with 14 remaining 50 - 14 = 36 = 18 x 2