This is the kind of thing a child will sometimes ask.  What we always do is crouch down so we're
on the same level as the child and then explain in clear, simple sentences that the white stuff in
the sky is the smoke blowing from fires ranging aboard the plane.  Kids love it!
You may want to try a different, more accurate approach:
The white stuff is ice.  It's condensed water vapor at high altitudes, eight miles
up or so, where the air is really cold, way below freezing.  As you know, a jet's
turbine engine sucks in air through a large opening on the front, then spews out the exhaust
through a small opening on the back side.  As water vapor gets squeezed through the engine it
gets denser.  As it emerges into the cold air behind the plane, it goes through a transition called
sublimation, in which it changes directly from vapor to ice without ever being a liquid.  How
very sublime!  A plane at a low altitude can have a liquid water trail behind the engine, but it's
not a contrail because it dissipates too quickly.  A real, proper contrail stretches across the sky.
Multiple factors are in play: temperature, humidity, wind.  The type of jet engine doesn't matter,
we're told by Boeing.
Of course, you won't remember any of this next time a child asks you about it.  Go with smoke.
          From "Why Things Are & Why Things Aren't (Joel Achenbach). Article contributed by Rtn. Mani