Add annotation on beamer slide, simulating hand-drawing?

Question

I would like to add on my beamer slides something that simulate as if I had been drawing a comment by hand. This would circle, come on top of text or formulas, and allows to insert text. Ideally, I could use this with the beamer overlay specification, i.e. add it only in a second step. See example below.

Is there anything that could bring me somewhere close to that?

\documentclass[english]{beamer}

\usepackage{babel}

\begin{document}
\begin{frame}{Make Titles Informative. }

Iintroduce here two theorems:
  \begin{theorem}
Algebra

1+2=3
\end{theorem}

\begin{corollary}
2+1=3
\end{corollary}

\end{frame}


\end{document}

Thanks!

Answer 1

If you got problems with tikzmark, here is a pure TikZ solution.

The command \mypos has 2 arguments: what to highlight and the name you would like to give to it. If the numbers are in math mode put $...$ in \mypos first argument.

\documentclass[english]{beamer}

\usepackage{babel}
\usepackage{tikz}
\newcommand{\mypos}[2]{\tikz[remember picture]{\node[inner sep=0pt, anchor=base](#2){#1};}}
\newcommand{\myund}{}
\begin{document}
\begin{frame}{Make Titles Informative. }

Iintroduce here two \mypos{theorems}{myth}:
  \begin{theorem}
Algebra

1+2=3
\end{theorem}

\begin{corollary}
2+1=\mypos{3}{mynum}
\end{corollary}
\begin{tikzpicture}[overlay, remember picture]
\node[draw, red, circle] (A) at (mynum){};
\draw[red] ([yshift=-2pt]myth.south west)--([yshift=-2pt]myth.south east) to[bend right] ++(1,-1) node[red, anchor=west] {Comment};
\draw[red] (A.south east) to[bend right] ++(1,-1) node[red, anchor=west] {Some text};
\end{tikzpicture}
\end{frame}

\end{document}

Answer 2

As pointed out by @cfr, tikzmark is very useful here. In addition, decorations.pathmorphing comes with random steps, which arguably looks as if you would have drawn it with your own claws, err, sorry, hands.

\documentclass[english]{beamer}
\usepackage{tikz}
\usetikzlibrary{tikzmark,decorations.pathmorphing}

\begin{document}
\begin{frame}{Make Titles Informative}

Introduce two \tikzmarknode{theorems}{theorems} here:
\begin{theorem}
Algebra

1+2=3
\end{theorem}

\begin{corollary}
2+1=\tikzmarknode[circle,draw=red,decorate,decoration={random steps,segment
length=1pt,amplitude=0.4pt}]{3}{3}
\end{corollary}
\begin{tikzpicture}[overlay,remember picture,decoration={random steps,segment
length=1pt,amplitude=0.4pt},red]
\draw[decorate] (theorems.south west) -- (theorems.south east)
to[out=-40,in=180] ++ (2,-1) node[right]{comment};
\draw[decorate] (3)
to[out=-40,in=180] ++ (2,-1) node[right]{some text};
\end{tikzpicture}
\end{frame}
\end{document}