Only high-IQ puzzlers can spot the hidden rider in a seemingly simplistic drawing of a horse within a tiny time limit, its makers have determined. A scribbled sketch shows an idyllic scene with a horse in the foreground grazing on a potted plant.

The animal is clearly fitted for a rider, with a pair of reins slotted around its head and a saddle on its back.

But it is riderless, with only one other figure visibly standing in the dark doorway of the nearby windmill in the background.

That dark…