Predicting Shelly's Excitement

Lucy has a pet turtle named Shelly. Every morning, Lucy feeds Shelly lettuce and carrots. Shelly loves to swim in her tank and bask under the special light that Lucy has set up. Lucy enjoys watching Shelly because she finds it calming. One day, Lucy notices that Shelly has been swimming more vigorously than usual. Lucy wonders why Shelly is acting this way.