What does the author mean by “the sand was boiling”?

PLEASE ANSWER THESE QUESTIONS. THANKS IN ADVANCE.

Once as a child he had sat upon a yellow dune by the sea in the middle of
the blue and hot summer day, trying to fill a sieve with sand, because some
cruel cousin had said, “Fill a sieve and you’ll get a dime!” And the faster he
poured, the faster it sifted through with a hot whispering. His hands were
tired, the sand was boiling, the sieve was empty. Seated there in the midst
of July, without a sound, he felt the tears move down his cheeks.

Questions :

What does the author mean by “the sand was boiling”?