CK-12

CK-12 offers free customizable digital textbooks, simulations, and practice problems covering middle and high school science. It functions as a Tappity alternative for budget-conscious homeschoolers needing flexible content they can remix. Unlike Tappity's curated elementary focus, CK-12 spans advanced topics and supports offline PDF downloads. The platform's flexibility is its main advantage, but it requires more parent curation to create weekly lesson plans comparable to Tappity's structure.