From Nookipedia, the Animal Crossing wiki
Speaking to Tortimer will result in the player's receiving a special event card. Every morning during this time, Copper will conduct Morning Aerobics at the Wishing Well with four other villagers. Players should have their event card stamped every time they attend an event. If a player gets the card stamped at fourteen Morning Aerobics sessions, Tortimer will reward him or her with a special Aerobics Radio, which enables the player to perform the exercises at home.
All exercises use the C-stick (yellow).