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Not exactly an advent calendar, but sort of! Target is currently offering the MindWare 12 Days of Mini Science Experiments. It is $24.99 and includes 12 days of mini science experiments.
From Target
This fun and eye-catching exploratory countdown is a twist on the traditional holiday countdown calendars. Kids can count down to fun with 12 days of mini science experiments. Each test tube contains step-by-step instructions and matericals for conducting experiments like making instant snow and an underwater eruption.