Methane is treated with a large excess of $\mathrm{Cl}{}_2$ in sunlight until all hydrogen atoms are substituted. The final organic product is

Each chlorination replaces one H with Cl, and the products themselves undergo further radical substitution in the presence of excess $\mathrm{Cl}{}_2$. The sequence $\mathrm{CH}{}_3\mathrm{Cl}\to \mathrm{CH}{}_2\mathrm{Cl}{}_2\to \mathrm{CHCl}{}_3\to \mathrm{CCl}{}_4$ ends at carbon tetrachloride.