The force of gravity acts towards the centre of the Earth, so how can these icicles grow almost horizontally? The answer requires the concept of the "centre of gravity" of an object, the point at which the object can balance on your finger tip. The icicles in the photograph initially grew vertically, formed as the snow on the roof slowly melted and refroze at the lowest point. However, the snow also slowly slid down the roof, to extend beyond the roof edge. To do this, the snow had to be a semi-fluid, so it could flow slowly but remain fairly rigid. As the leading edge of the snow moved further from the roof, the only way the centre of gravity could remain under the point of support was for the snow to curve backwards, resulting in the icicles tipping to become almost horizontal.