Conceptual Physics (12th Edition)

A traveling person's biological clock will tick more slowly than a stay-at-home-person's clock. A supremely irresponsible new parent could set out on a relativistic rocket ship, traveling at such a high speed that the $\gamma$ factor is 30. S/he could then return, two years older than when s/he left, only to find that the kid has aged by 60 years.