Ferns reproduce by alternation of generations. The large fern plant produces spores, which sprout into tiny, heart-shaped plants called Prothalia, which are usually only two to five millimeters wide. These tiny plants reproduce sexually, and their offspring are large fern plants. The generations alternate: large fern, tiny prothalia, large fern, tiny prothalia, etc.

Think if people went through the same alternation of generations. Imagine if your grandparents were human like you, but your parents and your children were mice. What a strange world that would be.