The transmission of sound can be illustrated by using a toy model consisting of an array of balls interconnected by springs. For real material the balls represent molecules and the springs represent the bonds between them. Sound passes through the model by compressing and expanding the springs, transmitting energy to neighboring balls, which transmit energy to their springs, and so on. The speed of sound through the model depends on the stiffness of the springs (stiffer springs transmit energy more quickly). Effects like dispersion and reflection can also be understood using this model. In a real material, the stiffness of the springs is called the elastic modulus, and the mass corresponds to the density. All other things being equal, sound will travel more slowly in spongy materials, and faster in stiffer ones. For instance, sound will travel 1.59 times faster in nickel than in bronze, due to the greater stiffness of nickel at about the same density. Similarly, sound travels about 1.41 times faster in light hydrogen (protium) gas than in heavy hydrogen (deuterium) gas, since deuterium has similar properties but twice the density. At the same time, "compression-type" sound will travel faster in solids than in liquids, and faster in liquids than in gases, because the solids are more difficult to compress than liquids, while liquids in turn are more difficult to compress than gases. Some textbooks mistakenly state that the speed of sound increases with increasing density. This is usually illustrated by presenting data for three materials, such as air, water and steel, which also have vastly different compressibilities which more than make p for the density differences. An illustrative example of the two effects is that sound travels only 4.3 times faster in water than air, despite enormous differences in compressibility of the two media. The reason is that the larger density of water, which works to slow sound in water relative to air, nearly makes up for the compressibility differences in the two media. In physics, a toy model is a simplified set of objects and equations relating them so that they can nevertheless be used to understand a mechanism that is also useful in the full, non-simplified theory. In "toy" mathematical models, this is usually done by reducing the number of dimensions or reducing the number of fields/variables or restricting them to a particular symmetric form. In "toy" physical descriptions, an everyday example of an analogous mechanism is often used to illustrate an effect in order to make the phenomenon easier to visualize. Some examples of "toy models" in physics might be: the Ising model as a toy model for ferromagnetism, or, more generally, as one of the simplest examples of lattice models; orbital mechanics described by assuming that the Earth is attached to the Sun by a large elastic band; Hawking radiation around a black hole described as conventional radiation from a fictitious membrane at radius r=2M (the black hole membrane paradigm); frame-dragging around a rotating star considered as the effect of space being a conventional "draggable" fluid. The phrase "Tinker-toy model" is also sometimes used in this context, and refers to a particular children's construction toy that allows objects to be built easily but somewhat unrealistically.