newenvironment vs newcommand?

Question

Can someone explain me this following strange bahavior?

The code is

\documentclass{amsart}

\newcommand{\pippo}{Pippo}
\begin{document}
\begin{pippo}  
\end{pippo}
\end{document} 

The output is

Pippo   

Answer 1

\begin{foo} is essentially \begingroup\foo and \end{foo} is \endfoo\endgroup if \endfoo is defined, and just \endgroup otherwise. So it is usually possible to call a command as an environment.