Use the makecell package
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\usepackage{makecell} % Use the makecell package
\begin{center}
\begin{tabular}{ | c | c | c |}
A Head & A Second Head & A Third Head\\
\hline
Some text & \makecell{Some really \\ longer text} & Text text text \\
\end{tabular}
\end{center}
https://tex.stackexchange.com/questions/2441/how-to-add-a-forced-line-break-inside-a-table-cell
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\begin{tabular}{|c|c|c|}
\hline
here&\vtop{\hbox{\strut top line}\hbox{\strut botline}}&more\\
\hline
x&y&z\\
\hline
\end{tabular}
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\documentclass{article}
\usepackage{tabularx}
\begin{document}
\begin{tabularx}{8cm}{|X|X|X|X|}
\hline
In this table & each column got the same & width & 2cm \\
\hline
As one could & easy see & the width is & to small for some of the cells \\
\hline
\end{tabularx}
\end{document}