Tokens: Why ChatGPT Can't Count the R's in 'Strawberry'

You see words. A language model sees tokens — chunks of text, usually a few characters each. Everything starts here. Day 2 of my AIFromZero series. Text gets shattered into tokens "unbelievable" → ["un", "bel", "iev", "able"] (4 tokens, not 1 word, not 12 letters) Before any "thinking", your text is chopped into tokens and each becomes a number the model processes. Why not words or letters? Letters : too fine — the model would relearn spelling everywhere. Whole words : too many — millions, plus every typo and name. Subword tokens : the sweet spot. Common words = 1 token; rare words split into reusable pieces. A fixed ~100k-token vocabulary covers any text. The ~4-chars rule (and why it costs you) In English, ~4 characters ≈ 1 token , or ~0.75 tokens per word. This is how everything is priced and limited: API bills are per token (prompt + reply). A "context window" (how much it can read at once) is measured in tokens — 1,000 tokens ≈ 750 words. Verbose prompts and long chat history burn tokens. Concise prompting is a real cost lever. The strawberry problem The model never sees s-t-r-a-w-b-e-r-r-y. It sees a token like straw + berry . The individual letters are buried inside tokens, so counting characters is genuinely hard for it. It's not dumb — it just doesn't read letters. Tokens are step 1 of everything Tokenize → turn each token into a vector (embeddings, next) → run through the transformer → predict the next token. Every LLM starts exactly here. 🔤 Type anything and watch it tokenize live: https://dev48v.infy.uk/ai/days/day2-tokens.html Day 2 of AIFromZero.