ether cleavage
Ethers are famously inert, but there is one thing that reliably breaks them apart: hot, concentrated, strong acid — especially hydroiodic acid (HI) or hydrobromic acid (HBr). Ether cleavage is the reaction that snaps the unbreakable C-O-C linkage, converting the ether into two pieces, typically an alcohol (or phenol) and an alkyl halide.
The trick the acid plays is to make the oxygen a leaving group. The strong acid first protonates the ether oxygen, turning the neutral, unreactive oxygen into a positively charged one whose departure as a neutral alcohol is now feasible. Then the halide ion (a good nucleophile, especially iodide) attacks a carbon attached to that oxygen and displaces it. The first cleavage gives one alcohol and one alkyl halide; with excess HI and heat, the alcohol that formed can be converted onward to a second alkyl halide.
Which carbon the halide attacks follows the same logic as substitution. If both carbons are methyl or primary, the halide attacks by SN2 at the less hindered carbon. But if one side is tertiary (or benzylic or allylic), the reaction can switch to SN1, since protonation gives a stable carbocation there. A useful special case: aryl-alkyl ethers (like anisole, methoxybenzene) cleave only on the alkyl side, because attacking an aromatic ring carbon by either SN1 or SN2 is not possible — so you get a phenol plus an alkyl halide, never an aryl halide.
Anisole (C6H5-O-CH3) heated with excess HI gives phenol (C6H5-OH) and iodomethane (CH3-I). Iodide attacks the methyl carbon by SN2; it cannot attack the aromatic ring carbon, so the phenol side stays intact.
Strong acid protonates the oxygen; the halide then cleaves at the carbon it can best attack.
HI and HBr cleave ethers; HCl is too weak and chloride too poor a nucleophile to do the job well. For an aryl ether, only the alkyl side ever breaks — you never get an aryl halide.