Creating a Safe, Stringy Paste for School Project

[pdxtester]

Hello, working on a school project with my nephew and looking to make a stringy paste like material hopefully using food related items and staying away from anything potentially harmful. Hopefully this would be somewhat thick with the ability to change by adding or reducing the amount of water or something else and wouldnt mold quickly if we stored in a jar with a somewhat thick consistency and possibly even a little stringy. Was thinking of using something that normal would gel and just adding less to not completely gel but not really sure where to start. Any help would be great, thanks!