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We get asked this question all the time. Bricks have core holes or recesses (called “frogs”) for several reasons. The most important is a principle called “keying in.”
When bricks are laid in a bed of mortar, the mortar squeezes into the holes which, when dried, locks the bricks into place. When the entire wall is built, the individual bricks are part of a wall system where a single brick cannot move without putting tension on the surrounding bricks. This gives the entire structure great strength.
Another reason is weight. A solid core brick is a fair bit heavier than a cored brick. Bricks with core holes are easier to transport, faster to lay, and put less stress on the foundation.
Bricks with core holes are also easier to manufacture since the heat from the firing more easily penetrates into the centre of the brick. This makes the brick stronger and quicker to produce.
Lastly, there is cost. Since cored bricks use less material they are less costly to produce.
So bricks have holes in them for strength, quality, weight reduction and economics, all of which are excellent reasons, indeed.