TITLE: Math games for car journeys
QUESTION [12 upvotes]: On long car journeys with kids we are all familiar with "I spy" or "Twenty questions". What math related games can one play on a car journey instead that are fun and offer similar variety?

REPLY [3 votes]: Try to find the prime factorization of every integer you come across - e.g. you see a 76 gas station, so you think: 
$$
76 = 2 \cdot 38 = 2 \cdot 2 \cdot 19 = 2^2\cdot 19 
$$