Not thinking harder. Thinking about whether the thinking holds up.
“Think critically” gets used as a generic scold, read more, question everything, don’t be naive without much specification of what the skill actually consists of. The research field built around the term treats it far more precisely: a definable set of sub-skills (interpretation, analysis, evaluation, inference, explanation, self-regulation) plus a disposition to actually use them, both separately measurable.
In 1990, a panel of experts convened by the American Philosophical Association produced a formal consensus definition, the Delphi Report, after finding that “critical thinking” had been used so loosely across education, philosophy, and psychology that research on it couldn’t accumulate without first agreeing what was being measured. Later work distinguished critical thinking skill (can you do it) from critical thinking disposition (are you actually inclined to do it under normal conditions). A person can possess the skill and still default to accepting claims uncritically most of the time, which is why training programmes that only teach the skill component often show disappointing real-world transfer.
Source: Facione, P.A. (2000). “The Disposition Toward Critical Thinking: Its Character, Measurement, and Relationship to Critical Thinking Skill.” Informal Logic, 20(1), 61–84 — extending Facione’s 1990 APA Delphi Report, the field’s original expert-consensus definition.