Cycling in Scotland the other day I took the chance to do some uphill freewheeling near Ayr.
Croy Brae, often nicknamed Electric Brae, is a 'gravity hill' - an optical illusion that fools you into thinking an up slope is a down slope, and that the laws of thermodynamics are being temporarily suspended.
It's on the A719, about seven miles of gentle climb and coastal views south out of Ayr. A sign warns you of slow traffic: lots of cars can't resist the temptation to stop, let off the handbrake, and roll magically against the gradient.
The illusion really is astonishing. This stretch of road clearly goes downhill into the trees, doesn't it?
Actually not, as the detailed stone plaque informs you in the lay by. In fact it's a quarter of a mile of 1 in 86 descent the other way, towards the camera.
If you stop on your bike 'down' in the middle of those trees and face towards the camera, you roll 'uphill', reaching a freewheel speed of about 10mph. (Conversely, of course, cycling the other way feels oddly strenuous for a 'downhill'.)
The strangest thing is how the illusion disappears the instant you lower your viewpoint (right), particularly when you're looking through a camera. With the surrounding hills out of your eye line, it's suddenly clear that the slope runs towards you. It's a remarkable demonstration of how subtle, but powerful, the subconscious effect is of the skyline on your mental spirit level.