The first verse in Parashat Teruma indicates that the Mishkan was built from voluntary donations. The ambiguity in the phrasing allows us to ask about the instructions: are you giving your heart, or is your heart giving you?

What is being given is generosity, and that is what the Mishkan is really made out of: aside from the physical materials, the real substance is the will of the giver-- appropriate for a place where people come to bring offerings to God.