Bees need pollen for food. Flowers have pollen. When bees land on flowers to collect pollen and nectar, they make a useful mess, spreading pollen from a flower’s stamen (where pollen is produced) to its stigma(where pollen is needed for fertilization). This ‘messy’ process is pollination which leads to fruits we can eat and seeds to grow new plants.
But bees don’t pollinate all flowers. Certain flowers have features that send “stop and visit” signals to bees. The bees land and the process of pollination benefit the flowers and the bees.