Teachers, who are adept in the subject, rack their brains and generate new worksheets with a support answer key to guide the instructor. There is no limit to the number of worksheets you generate for the class or tutorials and they can be used for distribution purposes.
There are many kinds of worksheets available but the best one is still the personally drafted one. This way one can easily check the level of difficulty that is offered to the kids according to their age and even check the progress level. Worksheets can be reused but it is always better to upgrade them from time to time.