Tom Yeung here with your Sunday Digest. Every parent knows that the concept of “gravity” comes naturally to children.  By 3 months, many infants can follow the path of a bouncing ball. Nine-month-olds throw food off tables to see the splat. By 2 years old, they’re at jungle gyms doing that with their whole bodies. My local urgent care clinic is routinely filled with grinning kids in arm casts. Fascinatingly, gravity is also found in investing.  Many assets trend toward a central value as if pulled by an inv ...