Corn silks get pollen on them that is carried by the wind from the tassels of corn in the area. It may be from the same plant or from plants a distance away.
If you are growing multiple kinds of corn, or a nearby neighbor has plants, you may want to insure you get what you expect.
There are a few ways to do this.
If it is just you growing more than one variety, time the seeding so that they don't mature at the same time. If both varieties mature at 90 days, for example, plant them about 2-3 weeks apart.
If one variety matures at 80 days and the other at 100, it is safe to plant them at the same time.
If you have a lot of land, you can simply plant them apart. I have read they need to be anywhere from 150 ft. to a mile apart. The corn in the picture above were about 6 ft. apart and generally upwind from a red dry corn. You can see there was a little cross pollination on the ear to the right, but it did take place.
If you are really into maintaining your seed supply to be true by preventing cross pollination, watch this video to learn how.
We're going to do this with our glass gem corn, to keep the seeds pure. There is a bed of dry corn about 120 ft. away, and we want to be sure they don't cross.
And, well... also because nerdy stuff like this is fun.