%%%------------------------------------------------------------------------------------------------------------
%%% 基于单词的模型的问题
\begin{minipage}[c]{0.35\linewidth}
\vspace{1em}
\begin{tikzpicture}

\begin{scope}
{\small
\node [anchor=west] (s1) at (0,0) {我};
\node [anchor=west] (s2) at ([xshift=1.0em]s1.east) {喜欢};
\node [anchor=west] (s3) at ([xshift=1.0em]s2.east) {\red{红}};
\node [anchor=west] (s4) at ([xshift=1.0em]s3.east) {茶};
\node [anchor=east] (s) at (s1.west) {$\textbf{s}=$};
}
\end{scope}

{
\begin{scope}[yshift=-2.0em]
{\small
\node [anchor=west] (t1) at (0.35em,0) {I};
\node [anchor=west] (t2) at ([xshift=0.8em,yshift=-0.0em]t1.east) {like};
\node [anchor=west] (t3) at ([xshift=1.0em,yshift=-0.0em]t2.east) {red};
\node [anchor=west] (t4) at ([xshift=1.0em,yshift=-0.1em]t3.east) {tea};
\node [anchor=east] (t) at ([xshift=-0.3em]t1.west) {$\textbf{t}=$};
}
\end{scope}

\draw [-] (s1.south) -- (t1.north);
\draw [-] (s2.south) ..controls +(south:0.5em) and +(north:0.5em).. (t2.north);
\draw [-] (s3.south) ..controls +(south:0.5em) and +(north:0.5em).. (t3.north);
\draw [-] (s4.south) ..controls +(south:0.5em) and +(north:0.5em).. (t4.north);
}%visible<4->

\begin{pgfonlayer}{background}
{
\node [rectangle,draw,thick,inner sep=0.2em,fill=white,drop shadow] [fit = (t3) (t4)] (problemphrase) {};
\node [anchor=north,text width=8em,align=left] (problemlabel) at (problemphrase.south) {\begin{spacing}{0.8}\scriptsize{``红 茶''为一种搭配，应该翻译为``black tea''}\end{spacing}};
}
\end{pgfonlayer}

\end{tikzpicture}
\end{minipage}