The Invisible Twins — Tom Mach

Amy and Becky, identical 10-year-old twins, find that their grandpa, a retired chemist, had accidentally discovered a formula which will make things disappear as well as another formula that will reverse the process. When grandpa is away, they apply the potion on themselves, hoping to have fun by being invisible while they play a trick on their friends. When they return, they apply the formula which was supposed to make them visible again, but it doesn't seem to work. What will they do? Will they miss out on the rest of their lives because of this?