The ES emini Future contract has a 1-to-1 pricing relationship with SPY ETF. When SPY is 94.00, the ES should be around 940.00 (a factor of 10). But when makes the ES and the SPY stay close together? Suppose the market closes on Friday and the SPY is 94.00 at the close, the ES continues to trade over the weekend. The ES price could go as high as 120.00 and as low as 80.00, for example. Eventually, the ES will be close to 940.00 again when the market opens for equities on Monday. Again, what causes the ES to eventually be close to SPY by the time the market opens on Monday?