To understand why plain water fails, look at how non-Newtonian fluids operate. Water molecules consist of simple, rigid bonds of hydrogen and oxygen ($H_2O$) that slide past each other with minimal friction. They do not tangle, interlock, or stretch under tension.
Slime forms only when microscopic chains hook together. In a standard craft setup, polyvinyl alcohol (PVA), the primary compound in clear glue, provides those chains. These extended polymers drift through the liquid like cooked spaghetti strands. When you introduce a cross-linking agent, borate ions link those individual strands together, creating a three-dimensional net that traps water molecules inside.
Without PVA or an equivalent synthetic polymer, cross-linking cannot occur. Salt added to plain water merely dissolves into sodium and chloride ions. Cornstarch produces an oobleck suspension that shows sheer-thickening properties, but it remains chalky, opaque, and brittle rather than elastic and clear. If a recipe contains zero long-chain polymers, it cannot yield genuine slime.
