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If you still have the liquid you cooked the pumpkins in, this can be reused. Otherwise, boil about 4 pints (2 liters) of fresh water. Use your hands or a large spoon. Do not mash the cubes into the jar. Instead, allow them to sit somewhat loosely. Leave about 1 inch (2.5 cm) of headspace or empty space in between the pumpkins and the top of the jar. Pour the water slowly to prevent it from splashing out and to allow it to fill the empty spaces in between the cubes of pumpkin. Make sure that approximately 1 inch (2.5 cm) of empty headspace still remains in each jar. Make sure that the lids are screwed on securely. If the lids are too loose, you will not be able to create a seal during the canning process. Heat the water to boiling. You must use a pressure canner when canning pumpkin. Use jar tongs to carefully place the jars on the rack inside the canner. Do not allow the jars to clink together as you move them. If the jars do collide, they may crack. Put the heat on high while the pressure valve is still open. Allow the steam to escape through this vent for ten minutes to purge the air inside. Close all the valves to allow pressure to build inside the canner. You will also need to set the weight, but the amount of weight will vary depending the time of pressure canner used as well as your present altitude.  If using a dial-gauge pressure canner, set the weight to 11 lb (4.99 kg)for altitudes up to 2000 feet (609.6 m). Add 1 lb (0.45 kg) of weight for every additional 2000 feet (609.6 m), stopping at 14 lb (6.35 kg). If using a weighted-gauge pressure canner, set the weight to 10 lb (4.54 kg) for altitudes up to 1000 feet (304.8 m). For any higher altitude, set the weight to 15 lb (6.8 kg). If using quart-size (liter-size) jars, however, you will need to process the pumpkin for 90 minutes. Turn off the heat after the jars have finished processing. After the pressure drops to zero, wait three additional minutes and open the vent to release the steam. Open the canner and lift the jars out with jar tongs. Set them to dry and cool in a draft-free area. Do not allow them to touch or bump into each other. After the jars have cooled overnight, press on the center of each lid to check the seal. If it does not move, the jars have been sealed properly. If the center button "pops" or moves up and down when pressed, the jars have not been sealed and are not safe for long-term storage. A basement or refrigerator works best.

Summary:
Boil water. Pack the pumpkin cubes into the jars. Pour the boiling water into the jars. Put the lids onto the jars. Fill a pressure canner with 3 inches (7 2/3 cm) of water. Place the jars in the canner. Cover the canner but allow it to vent. Set the weight and close the vents. Process the jars for 55 minutes. Turn off the canner and let cool. Remove the jars and let cool. Check the seals. Store sealed jars in a dark, cool area.