Corn in Baby Sign Language looks like you are holding a cob of corn with both open, curved hands, and munching away (sans the stream of butter dripping down your shirt). Make the sign by holding an imaginary cob of corn with both hands in front of your mouth, and nibbling along the corn with your mouth, turning your hands twice. A one-handed version (if you're carrying your baby in your other arm), is to get your pointer or index finger, hold it horizontally across your mouth, and turn it twice, again as if eating a cob of corn.
Corn is a favorite of many babies, who enjoy being able to gnaw on something by themselves. If corn is a favorite, keep a few cobs cooked and in the fridge, and teach your baby the sign so he can ask for corn whenever he is hungry.
Why not plant a little garden with your baby and watch the corn grow? This sign can be introduced with meals or through visual cues outdoors.