Opinion of BREYER, J.
Rocky Mountain Low-Level Radioactive Waste Compact Board et al. as Amici Curiae 16–18. This rotational approach is surely a sensible solution to the problems caused by the widespread existence of low-level nuclear waste and the political unpopularity of building the necessary facilities to house it. See id., at 13–16; New York v. United States, 505 U. S. 144, 149–151 (1992).
The only contrary evidence—i.e., that indicates that North Carolina did not bear ultimate funding responsibility—consists of the fact that the Commission voluntarily advanced North Carolina nearly $80 million between 1988 and 1998 in order to help it defray its costs. Second Report 16. The Court believes that this “course of performance” demonstrates that, once the Commission turned off its monetary spigot, North Carolina was no longer required to do anything further. Ante, at 14–15. But why? If I advance my builder half the cost of a building, I have not thereby promised to advance him the whole cost. This is particularly true when the contract says I am responsible for none of the cost of the building. At the very least, something more in the circumstances would have to show that additional expenditure had become a reasonable expectation.
In this case, nothing suggests that North Carolina could reasonably expect further financing assistance. Indeed, I can find nothing in the majority’s opinion, or the record, that suggests that the Commission or the other Compact States intended to let North Carolina off the hook. And numerous documents indicate precisely the opposite—that despite the Commission’s funding assistance, North Carolina was still responsible for funding the project. See, e.g., App. 63 (Resolution (Feb. 9, 1988) (“The Commission, although not obligated to do so under the Compact,” provides funding for North Carolina)); id., at 215 (Letter from Richard S. Hodes, M. D., Chairman, Southeast Compact Commission, to James B. Hunt, Governor of North Caro-
