Pineapple is signed by taking your dominant hand and forming the letter 'F' in ASL: hold your index finger and thumb together while the rest of your fingers are splayed out. Then hold the circular space between the two fingers to your dominant eye and pivot it around the eye a couple times, as if to suggest one of the many "eyes" on a pineapple, while the splayed fingers suggest the spiny leaves sticking out at the top of this tropical fruit.
Teach your baby this sign in conjunction with the other fruit signs. It will be fun as your baby learns to sign exactly which fruits are his or her favorite.
Teach the pineapple sign to your baby when it is served as part of a dessert tray, or when you are out shopping for fruits.