Science fiction is a genre that combines science and fiction, constrained by the physical laws of nature. It evolved in response to fantasy, which allows far-fetched assumptions like unicorns. The first science fiction novel was Frankenstein by Mary Shelley in 1818, which used scientific rationale and religious ideas to make improbable events seem possible. Jules Verne also wrote scientifically inspired fiction like 20,000 Leagues Under the Sea, with elements close to reality that have since become inventions. Science fiction established itself as a film genre in the early 20th century and grew in popularity after events like the atomic bombings raised interest in speculative futures.