Cell membranes and transport: explain question

A researcher prepared two types of artificial lipid bilayers: one consisting only of phospholipids, and the other containing phospholipids and cholesterol. The fluidity of each bilayer was measured at 10°C and 40°C. At 10°C, the bilayer with cholesterol had higher fluidity than the bilayer without cholesterol. At 40°C, the bilayer with cholesterol had lower fluidity. Explain these observations in terms of the fluid mosaic model and the roles of cholesterol and other components of cell surface membranes.