All the different kinds of Corn
Talk to your class about corn. Then show them all of the different things that
are made from corn (corn kernels, cornstarch, cornmeal, popcorn). Ask them if
they think these all came from the same plant. Then get a whole cob of corn
(often sold as Squirrel food) and let them see the source.
What happens to the eggs
Get several raw eggs and place them into different liquids (vinegar, cola, water,
kool-aid, etc.) Ask your kids what they think will happen to each egg. Put them
in a corner then in a couple of days look at the different effects the substances
had on the egg.
Grow some Corn
Get dried corn on the cob from a store. Then pluck all of the kernels off of
the cob. Next put the cob into a clear container and wait about one to two weeks.
New corn sprouts should begin to form. You can plant this and grow corn.
What Farm Animals Eat
Go to a local feed store and buy different things that farm animals eat and bring them so that your class can explore them. 
Milk a Cow
Talk to your children about where milk comes from. Then fill a latex glove full
of milk and poke small holes in one or two of the fingers. Now the children
can see how it is to really milk a cow.