why does ``grape`` have ``ignoreAir``?

function doAirDensity(pixel) {
            if (pixel.del) {return}
            var info = elements[pixel.element];
            if (!info.ignoreAir && info.density !== undefined && info.movable === true && info.density < airDensity) {
                // Air Density
                // if the pixel's state + ">" + newPixel's state is in validDensitySwaps, and the pixel's density is larger than the newPixel's density, swap the pixels
                if (validDensitySwaps.gas[info.state]) {
                    // chance depending on the difference in density
                    if (Math.random() < (airDensity - info.density)/(airDensity + info.density)) {
                        tryMove(pixel,pixel.x,pixel.y-1);
                    }
                }
            }
        }

function doAirDensity(pixel) {
            if (pixel.del) {return}
            var info = elements[pixel.element];
            if (!info.ignoreAir && info.density !== undefined && info.movable === true && info.density < airDensity) {
                // Air Density
                // if the pixel's state + ">" + newPixel's state is in validDensitySwaps, and the pixel's density is larger than the newPixel's density, swap the pixels
                if (validDensitySwaps.gas[info.state]) {
                    // chance depending on the difference in density
                    if (Math.random() < (airDensity - info.density)/(airDensity + info.density)) {
                        tryMove(pixel,pixel.x,pixel.y-1);
                    }
                }
            }
        }

function doAirDensity(pixel) {
            if (pixel.del) {return}
            var info = elements[pixel.element];
            if (!info.ignoreAir && info.density !== undefined && info.movable === true && info.density < airDensity) {
                // Air Density
                // if the pixel's state + ">" + newPixel's state is in validDensitySwaps, and the pixel's density is larger than the newPixel's density, swap the pixels
                if (validDensitySwaps.gas[info.state]) {
                    // chance depending on the difference in density
                    if (Math.random() < (airDensity - info.density)/(airDensity + info.density)) {
                        tryMove(pixel,pixel.x,pixel.y-1);
                    }
                }
            }
        }

function doAirDensity(pixel) {
            if (pixel.del) {return}
            var info = elements[pixel.element];
            if (!info.ignoreAir && info.density !== undefined && info.movable === true && info.density < airDensity) {
                // Air Density
                // if the pixel's state + ">" + newPixel's state is in validDensitySwaps, and the pixel's density is larger than the newPixel's density, swap the pixels
                if (validDensitySwaps.gas[info.state]) {
                    // chance depending on the difference in density
                    if (Math.random() < (airDensity - info.density)/(airDensity + info.density)) {
                        tryMove(pixel,pixel.x,pixel.y-1);
                    }
                }
            }
        }