Bee-pollinated flowers tend to be yellow or blue, often with ultraviolet nectar guides and scent. Nectar and/or pollen are offered as rewards in varying amounts. The sugar in the nectar tends to be mostly sucrose. There are different types of bees which differ in size, tongue length and behaviour (some solitary, some colonial). Some plants can only be pollinated by bees because their anthers release pollen internally, and it must be shaken out by buzzing ("sonication"). Bumblebees are the only animals that do this.