[header] How to build a hydrometer [title] Seal off an end of a soda straw. [step] You can plug the end with modeling clay, plasticine (play-doh or cranium clay), or a wad of chewed gum. Be sure to use something that won't let water seep through it, and use a piece large enough not to let water seep past it. 
[title] Fill a narrow jar or graduated cylinder partially with water. [step] If using a graduated cylinder, fill it to the 100 milliliter line.