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Overview

In FHE-enabled smart contracts, you often need to perform operations between encrypted values and regular plaintext values. Trivial encryption is the operation of converting a plaintext value into an encrypted format that can interact with other encrypted data. This conversion is done using the FHE.asEuint family of functions, which take standard Solidity types and transform them into their encrypted counterparts. 
uint16 number = 2;
euint16 encrypted_number = FHE.asEuint16(number);

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Privacy Considerations

Trivially encrypted values are not confidential - they are merely a tool to enable interaction between encrypted and non-encrypted types. The original plaintext value remains visible to anyone observing the blockchain, just in a different format. This is a crucial concept to understand when developing confidential contracts. Any value that is trivially encrypted (e.g. encrypted_number) should be treated as public information, even though it uses the same data type as truly encrypted values.
euints are only confidential when they are formed from encrypted inEuint inputs, which are encrypted off-chain. Learn more in the Data Evaluation guide.
When two trivially-encrypted numbers are combined in an FHE operation, the result is still not confidential, because an observer can keep track of the calculations.

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Example

function doSomeCalculations(InEuint16 calldata input) {
    // public
    euint16 number2 = FHE.asEuint16(2);
    euint16 number3 = FHE.asEuint16(3);
    euint16 number5 = FHE.add(number2, number3);

    // confidential
    euint16 encInput = FHE.asEuint16(input);
    euint16 encMul = FHE.mul(encInput, number5);

    // public
    euint16 eFalse = FHE.asEbool(false);

    // Observer knows that result is encInput, but not what the value is
    euint16 result = FHE.select(eFalse, encMul, encInput);
}