Classification (Odd One Out)
🔒 Log in to trackWord odd one out
🔒 Log in to trackFour words from overlapping fields; one breaks the shared category or function. CGL recycles the same pools:
| Pool | Common odd ones |
|---|---|
| Animals | bat/mammal among birds, whale among fish, snake among legged reptiles |
| Computer parts | monitor/speaker (output) among input devices, or vice versa |
| Geography | sea/ocean (saline) among freshwater bodies |
| Science | a symptom (fever) among diseases, a unit that is not base SI |
| Occupations | an amateur activity among professions |
The exam expects school-level GK only. If two words look odd at category level, check a property level (even/odd, edible/non-edible, living/non-living, input/output).
Detailed notes
What is word classification?
You get four words. Three belong to one group; one does not. The group can be a category (all birds), a use (all writing tools), a property (all liquids), a general-knowledge set (all state capitals) or a meaning (all words meaning 'happy'). The exam needs only school-level knowledge - things taught up to Class 10.
Everyday example: pen, pencil, chalk, eraser. Pen, pencil and chalk are used to write. An eraser removes writing. So the eraser is odd.
Step-by-step method
- Say the most obvious group. "These are all stationery items." If all four fit, this level gives no answer.
- Go one level deeper. "Which of them write?" Now three fit, one does not.
- Check the rule on all four. If two words break it, choose a different rule.
- Prefer the most precise rule. "Three are metals that are pure elements; brass is a mixture (alloy)" is more precise than "all are shiny".
The five rule families
1. Category (biology is the favourite).
- Bat among birds → bat is a mammal.
- Whale, dolphin, seal are mammals; shark is a fish.
- Frog, toad, salamander are amphibians; lizard is a reptile.
- Potato, carrot, radish grow under the ground; tomato grows above.
2. Function / use.
- Thermometer, barometer, hygrometer measure something; a microscope magnifies.
- Keyboard, mouse, scanner are input devices; monitor, printer, speaker are output devices.
3. Property.
- Liquid vs solid (milk, water, juice vs butter).
- Pure element vs alloy (gold, silver, platinum vs brass, bronze, steel).
- Conductor vs insulator (copper, aluminium vs rubber, wood).
4. General-knowledge sets.
- State capitals: Lucknow (Uttar Pradesh), Patna (Bihar), Bhopal (Madhya Pradesh). Indore, Kanpur, Kochi are big cities but not capitals.
- Musical instruments: tabla, dholak, mridangam are percussion (struck); sitar, veena, violin are string instruments.
- SI base units: kilogram (mass), metre (length), second (time). Litre is a common unit of volume but not an SI base unit.
5. Meaning (synonym group). Three words mean nearly the same; one means the opposite. Happy, glad, cheerful vs gloomy. Abundant, plentiful, ample vs scarce.
Worked example
Odd one: Thermometer, Barometer, Hygrometer, Microscope.
- Level 1: all are scientific instruments - no answer.
- Level 2: thermometer measures temperature, barometer measures air pressure, hygrometer measures humidity. A microscope does not measure; it makes tiny things look bigger.
- Answer: Microscope.
Traps to avoid
- Spelling or length traps. Do not pick a word just because it is longer or starts with a vowel, unless no meaning rule works.
- Two possible answers. If both A and B look odd, your rule is too loose. Find the rule that leaves exactly one.
- Common-use confusion. A tomato is cooked like a vegetable, but the question's rule may be "grows under the ground". Read the other three words to find the rule.
Quick revision
| Family | Typical odd one |
|---|---|
| Birds vs mammal | Bat |
| Mammals vs fish | Shark |
| Amphibians vs reptile | Lizard |
| Measuring instruments | Microscope |
| Input devices | Monitor / Printer |
| Pure metals vs alloy | Brass, Bronze, Steel |
| Percussion vs string | Sitar, Veena |
| Synonym group | The antonym |
Types of questions asked
Every way this subtopic shows up in exams — how to recognise it, the formula or logic to use, and a solved example.
Type 1: Category break (animals, plants, living things)very common3 practice Q
Four names from nature: birds, fish, reptiles, vegetables, trees. One belongs to a different biological group.
- Name the biological group of each item (mammal, bird, fish, reptile, amphibian).
- Three will share a group.
- The item from another group is the answer.
Why it works: the setter picks a lookalike from a neighbouring group (bat among birds, shark among whales).
Example: Find the odd one: Sparrow, Crow, Parrot, Bat.
Sparrow, crow and parrot are birds. A bat is a mammal (it feeds its young on milk). Answer: Bat.
Type 2: Function / use breakvery common3 practice Q
Four tools, devices or instruments. Three do the same job; one does a different job.
- Say what each item is used for.
- Group the three with the same job.
- The item with the different job is odd.
Why it works: items from the same place (office, lab, computer) often have different uses.
Example: Find the odd one: Pen, Pencil, Chalk, Eraser.
Pen, pencil and chalk are used to write. An eraser removes writing. Answer: Eraser.
Type 3: Property break (state, material, natural/man-made)common2 practice Q
All four are the same kind of thing, but differ in a property such as solid/liquid, pure/mixed, natural/man-made.
- Confirm the obvious category fits all four.
- Check properties: state of matter, element or mixture, conductor or insulator, living or non-living.
- The item with the different property is the answer.
Why it works: when category is common to all, the setter has used a property.
Example: Find the odd one: Gold, Silver, Platinum, Brass.
Gold, silver and platinum are pure metals (elements). Brass is an alloy of copper and zinc. Answer: Brass.
Type 4: General-knowledge set (capitals, instruments, units)common3 practice Q
Words from a GK list: cities, state capitals, musical instruments, units of measurement, planets.
- Recall the school-level fact for each word.
- Find the set three belong to (state capitals, percussion instruments, SI base units).
- The word outside that set is odd.
Why it works: the odd word is from the same topic but not from the same list.
Example: Find the odd one: Lucknow, Patna, Bhopal, Indore.
Lucknow, Patna and Bhopal are state capitals (UP, Bihar, MP). Indore is a large city of MP but not its capital. Answer: Indore.
Type 5: Meaning group (synonym set with one opposite)common2 practice Q
Four adjectives or verbs; three mean nearly the same thing.
- Find the common meaning of most words.
- The word with the opposite (or unrelated) meaning is odd.
Why it works: three synonyms form the group; the antonym is planted as the outsider.
Example: Find the odd one: Happy, Glad, Cheerful, Gloomy.
Happy, glad and cheerful all mean joyful. Gloomy means sad. Answer: Gloomy.
Shortcut tricks
⚡ Two-level drill: category, then property
First find the broad category word shared by three. If the fourth also fits that category, switch to a binary property (salt/fresh, input/output, base/derived, male/female) — one of the four will be the only one on the other side.
Example: Q. Snake, Crocodile, Lizard, Whale
Sol. Category 'reptile' fails to exclude whale (a mammal) cleanly? It does exclude whale — but verify: snake, crocodile, lizard are reptiles, whale is a mammal → Whale. If instead the list were Snake, Crocodile, Lizard, Turtle, all four are reptiles, and the property level (venomous/non-venomous) would decide.
Category first; if 2 candidates remain, a binary property splits them.
Where students lose marks
Choosing by rarity ('turtle is uncommon') instead of a structural difference.
Missing the functional split in device questions — CGL loves input vs output.
Overlooking that a word can belong to two categories (bat: mammal AND flying animal); pick the option where all others share a category that the odd one cannot join.
Practice sets — 17 questions
Sets of 10, mixed across the question types above. Each answer comes with a step-by-step explanation.
Topic test · 10 questions
Suggested time 5 min · wrong answers go to your mistake notebook automatically.