Only use Gatorade or G2 bottles for the sensory bottles they are a heavier plastic so they hold up better and keep a unison look so everything looks good. They come in different sizes you can find smaller ones for small hands if you need to.
Make sure to ALWAYS seal your bottles securely! I cannot emphasize this enough. Unless you want your bottle opened and the contents poured out. SEAL IT… with glue. Duct tape can be peeled off by a determined kiddo. Glue is much harder for them to open up.
Take two bottles. Fill one with water and one with clear shampoo or hair gel. Add a marble to each bottle, then secure the lid closed. The children can observe how the marbles move through different liquids.
For a list of ALL my amazing DISCOVERY BOTTLE IDEAS click the link or the image below