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\setbeamercolor{postit}{fg=black,bg=yellow}
\begin{beamercolorbox}[center,colsep*=1pt]{postit}
text
\end{beamercolorbox}

- black text in a yellow-filled box
- the box is margine-to-margine wide
- text is centered
- the upper/lower border of the box is 1pt above/below the text


\setbeamercolor{postit}{fg=white,bg=blue}
\begin{beamercolorbox}[center,colsep*=4pt,rounded=true,shadow=true]{postit}
text
\end{beamercolorbox}

- white text in a blue-filled box
- the box is margine-to-margine wide
- text is centered
- the upper/lower border of the box is 4pts above/below the text
- the box has rounded corners
- the box has shadow

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\framebox{\parbox{3cm}{the symmetry of\\ the ground state}}

- the border lines drawn around the 3cm wide box containing the given text
- the height of the box is determined by the text inside


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\fbox{text}

- the border lines drawn around the box of size determined by the text inside

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\usebeamercolor{beamer color}\colorbox{yellow}{\color{blue}text}

- blue text in a yellow-filled box
- the size of the box is determined by the size of the text