  The success of language models has inspired the NLP community to attend to
tasks that require implicit and complex reasoning, relying on human-like
commonsense mechanisms. While such vertical thinking tasks have been relatively
popular, lateral thinking puzzles have received little attention. To bridge
this gap, we devise BRAINTEASER: a multiple-choice Question Answering task
designed to test the model's ability to exhibit lateral thinking and defy
default commonsense associations. We design a three-step procedure for creating
the first lateral thinking benchmark, consisting of data collection, distractor
generation, and generation of adversarial examples, leading to 1,100 puzzles
with high-quality annotations. To assess the consistency of lateral reasoning
by models, we enrich BRAINTEASER based on a semantic and contextual
reconstruction of its questions. Our experiments with state-of-the-art
instruction- and commonsense language models reveal a significant gap between
human and model performance, which is further widened when consistency across
adversarial formats is considered. We make all of our code and data available
to stimulate work on developing and evaluating lateral thinking models.
