Let $\overline{\mathbf{M}}_{0,n}$ be the moduli space of stable $n-$pointed smooth rational curve of genus zero and $\overline{\mathbf{U}}_{0,n}$ the universal family described by $\pi_n:\overline{\mathbf{U}}_{0,n}\longrightarrow\overline{\mathbf{M}}_{0,n}$ with the n disjoints sections $\sigma_i: \overline{\mathbf{M}}_{0,n}\longrightarrow \overline{\mathbf{U}}_{0,n}.$ Joachim Kock in "An invitatation to quantum cohomology" Example 1.5.11 gave the relationship between a boundary $\mathbf{F}_n$ of $\overline{\mathbf{M}}_{0,n}$ with the boundary $\mathbf{F}_{n+1}$ of $\overline{\mathbf{M}}_{0,n+1}$ by the following formula $$ \mathbf{F}_{n+1} = \varepsilon^*\mathbf{F}_n + \sum_{i}\sigma_i.$$ Can someone explain me more how to get the formula?