Some objects are hard because they are big. These are hard because they lie to you about where their weight is.
A brick is easy. Its mass sits exactly where it looks like it sits, so the visual middle and the mass middle are the same line.
Everything below breaks that agreement.
Cones, carrots, drumsticks, pears stood upright.
Volume scales with the cube of size, so the fat end carries far more weight than its length suggests. Cut a cone halfway up and the top piece is one eighth of the whole. For equal mass the blade lands about a fifth of the way up from the wide end, which looks absurd and is correct.
The miss: people cut near the visual middle and hand the fat half a third more weight.
Peaches, avocados, olives, mangoes.
The stone is dense, off-centre, and invisible. You are asked to halve an object while a significant fraction of its mass hides somewhere you cannot see, and its position varies from fruit to fruit.
The miss: unpredictable in direction, which makes it the only shape on this list you cannot learn your way out of. You can only cut and find out.
Bottles, jugs, anything with a neck and a base.
The glass is thick at the base and thin at the neck, so most of the weight sits in the bottom third while most of the length sits above it.
The miss: consistently high. People cut near the middle of the silhouette and leave the base half far heavier.
Pears on their side, gourds, fish.
Mass concentrates in the belly. The widest cross section is not the midpoint of the length, and the belly's contribution grows with the square of its radius.
The miss: toward the tail, every time.
Rope, celery, baguettes.
These are the easy ones. A uniform cross section means length and mass track each other, so the visual middle is the mass middle, and being off by a centimetre costs almost nothing.
The miss: small, and mostly the leftward drift everyone has.
Find the widest cross section, then move the blade toward it. The wider the shape gets at any point, the more disproportionate its weight there, because area grows with the square of the radius while your eye reads width in a straight line.
Halfsies is the casual daily split-in-half cutting game: three objects a day, one cut each, both halves weighed to the gram and scored out of 300. The objects rotate, so the shapes on this list turn up often enough to learn.