An Equation for Waves

They lie in bed trying to work out
an equation for waves, a way
to present blue and the troughs
a wave makes as it grows and moves through,

a knowledge of shape over time.
"Sinusoidal," one says. "Let x,"
the other replies, be a margin
"and y the height of the crest before

it falls down." Betweentimes their backs
are an unquiet surface, their bellies,
like swell against sand, water

sinking past grains, grains lifted
by waves, the only unknown
the depths where they form and return.