Concept:
Kirchhoff's Current Law states that the algebraic sum of currents meeting at a junction is zero.
Step 1: At any junction in an electrical circuit, charge cannot accumulate.
Step 2: Therefore, the total current entering the junction must be equal to the total current leaving the junction.
\[
\sum I_{\text{in}}=\sum I_{\text{out}}
\]
Step 3: This is possible because electric charge is conserved.
Therefore, Kirchhoff's current law is based on:
\[
\boxed{\text{Conservation of charge}}
\]