What did God promise to open if the people brought the full tithe?

Answer: The windows of heaven

"Bring the full tithe into the storehouse... Test Me in this... See if I will not open the windows of heaven and pour out blessing."

“Bring the full tithe into the storehouse, so that there may be food in My house. Test Me in this,” says the LORD of Hosts. “See if I will not open the windows of heaven and pour out for you blessing without measure.”— Malachi 3:10 (BSB)