By insects: When a bee, butterfly, etc. try to get the sticky sweet pollen inside a flower, the pollen catches on to the insect, too. Then when the insect goes to the next flower, the pollen in pollinates. Note that this is <em>not</em> on purpose.
2. Another way plants are pollinated is by the wind. The wind picks up pollen from one plant and blows it onto another. Note that this is by nature.