Mochila means “backpack” in Spanish. In the days of the Pony Express, riders would carry the mail in a satchel thrown over the saddle and secured in place by the weight of the rider. That satchel was referred to as the mochila. To deliver mail from Missouri to California in just 10 days, the riders would change horses every 10 miles, quickly transferring the mochila from one horse to another at each station. This Google doodle commemorated the 155th anniversary of the Pony Express on April 14, 2015.