... which means the writer gives first-hand information, or if the sourceis a secondary source, which means the writer gives second- hand information. Another point to consideris whether the source ... bladder,so they use their forward movement to control their verticalposition. The tail is like the shark’s propeller. The shark swings itback and forth to move forward. In an airplane, this forwardmovement ... bladder,so they use their forward movement to control their verticalposition. The tail is like the shark’s propeller. The shark swingsit back and forth to move forward. In an airplane, this forwardmovement...