When Australian pupils return home from school during the spring term, they often look like they've been involved in an all-day wrestling match. They may have appeared fine as they left, but when their parents see them again they are drenched with sweat and several kilos lighter.
Every year, the temperatures outside have been hot enough for pupils to fry eggs on their satchels. Yet most government schools still lack any air conditioning - other than an open window.
It has long been thus. Despite the fact that Australia endures its hottest weather in late January and early February, this is when the kids return to school.
Many European countries shut their classrooms for eight or more weeks over the summer, but not here: a break of a month just before Christmas is considered quite enough time for children to spend annoying the adults.
The worst of the heat is now over now. But many Australian pupils will still be wheezing and breathless after their walk to class. One in five children here is now overweight or obese - double the proportion a decade ago.
Australians are following Americans, in that increasing numbers of fat teenagers are being diagnosed with type 2 diabetes, historically a disease that rarely affected people younger than 45. Complications of type 2 diabetes include heart disease, stroke, kidney failure and even blindness.
Overcoming the problem of too many fat kids (and their parents) depends on the ability of researchers to disentangle complex environmental, social, physical and economic factors. How these contribute to the epidemic and how they affect people of different socio-economic status across the age spectrum are among the issues still to be resolved.
The obvious explanation why children in poorer areas are more likely to be overweight or obese than their counterparts in wealthier suburbs is diet.
But that's not the whole story: researchers say parenting skills are important. The education level of the mother is related to how much TV her children watch: if your mother did not complete high school, you watch much more.
And, of course, there is also the direct impact of television and computer games on young people, which can result in them spending more time inside.
In one study involving 1,200 families from poor and wealthy suburbs around Melbourne, the activity of children aged five to six was compared with a group of 10 to 12-year-olds using matchbox-sized "accelerometers" that monitored their movements.
One striking finding was that younger children spent 200 minutes a day in moderate exercise. But the older group spent only half that time in moderate physical activity and 20 minutes of more vigorous movement.
The shift in priorities arose because the older children had more homework, used computers more and watched more TV.
One old timer here once remarked that he liked kids because "they always rush outside as soon as they get what they want". Not any more, sadly. And it's not just because it's so hot out there.