Bees make two well known substances: bees wax and honey. They also make a sticky substance called propolis or bee glue. It is made from the sap of local plants mixed with their saliva. It is used to seal up cracks in the space that the bees are using for their hive. This can be a problem for the beekeeper since it makes it difficult opening the hive and removing individual frames. Some species of bees also use it to keep other insects out of their hives by spreading it around the entrances.