This article contains material that is not in the current version of Tekkit.
The content you see here is not available if you are using the current version.
To create Water Cans, a Rainwater Tank needs to be supplied with Empty Cans or Wax Capsules. Using a Wax Capsule will create Water Capsules which are functionally identical.
Water Cans or Capsules can also be used to supply Fermenters and Carpenters or Biogas Engines with water via those machine's manual input/output slot.