This protocol is used to prepare 10% Tween 20. Tween 20, when added to aqueous solutions acts as a surfactant, decreasing the surface tension of the solution. It also punches holes in membranes, allowing easier penetration of macromolecules into cells and organelles, and reduces non-specific binding of antisera in immunohistochemistry. Note: Research reported in this publication was supported by the National Institute Of Mental Health of the National Institutes of Health under Award Number U19MH114830. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.