What kind of special call can a contract make to another contract that stops both contracts from changing their saved data during that call?

The special call a contract can make to another contract that stops both contracts from changing their saved data during that call is called a `STATICCALL`. A `STATICCALL` is a specific type of message call opcode within the Ethereum Virtual Machine, which is the runtime environment where smart contracts execute. It is fundamentally designed to be read-only, meaning it strictly enforces that no state modifications can occur throughout its execution. "Saved data" refers to a contract's "state variables," which are pieces of information stored persistently on the blockchain by the contract, such as balances, counters, or entries in a mapping. "Changing their saved data" means altering the values of these state variables or performing any operation that writes to the blockchain, including creating new contracts or triggering self-destruction. This restriction applies not only to the immediate recipient of the `STATICCALL` but also to any subsequent internal calls initiated within that `STATICCALL`'s execution context. If any contract attempts a state-modifying operation during a `STATICCALL`, the entire call will immediately fail and revert, ensuring no state changes are committed. This mechanism is a critical security feature, allowing a contract to safely query information from another contract without any risk of unintended side effects or state alterations, even if the called contract is malicious or contains bugs. For instance, if Contract A needs to retrieve a balance from Contract B and wants to guarantee that Contract B cannot spend or transfer any tokens during this retrieval, Contract A would use a `STATICCALL` to ensure this read-only guarantee.