Scratch Coding Guide: From the First Block to Their Own Projects

ScratchJr is the younger sibling of Scratch, built for kindergarten and the first years of primary school. Its blocks are icons with no words on them: an arrow pointing right, an arrow pointing up, a speech bubble. A child who cannot read yet can still put a program together. Blocks snap together from left to right, the same direction as reading, and a sequence usually opens with a yellow block showing a green flag or a tapping finger.

A typical ScratchJr project is a short story. A character walks across a park, jumps, then says hello. Children can draw their own character in the paint editor, record their voice and add a new page for the next scene. All of this builds a sense of sequence and cause and effect through touch, which is exactly the groundwork they will lean on in the full version of Scratch.

Two signs usually show a child is ready to move on. They read short labels comfortably, and they want to make a game that counts something. ScratchJr has no score variable, so that wish becomes a natural doorway into Scratch. Some second graders are ready early, while others prefer to finish a few more ScratchJr stories first, and both paths are perfectly normal.

The yellow blocks with a rounded top, such as when green flag clicked, are called hat blocks. They live in the Events category and always sit at the top of a stack, because their job is to wait for something to happen and then run everything beneath them. Besides the green flag there are hat blocks for a key being pressed, a sprite being clicked, a backdrop switching and a message arriving.

Children often build three correct blocks and forget the hat. The program then runs only when the stack itself is clicked and ignores the green flag entirely. The opposite slip turns up just as often: one sprite carries two green flag stacks that fight over its position, so the movement looks jerky. Counting the hat blocks on each sprite is a quick way to read what a project actually does.

A small project that exercises this idea: a cat that steps forward when the right arrow key is pressed, backs up on the left arrow and greets the player when clicked. Three different hat blocks give three behaviours that can be tested one by one. From there a learner sees that a Scratch program is really many small stacks, each waiting for its own trigger, and that the red stop button halts them all at once.

Each sprite can hold several costumes, slightly different pictures such as a cat with its front leg forward and a cat with its back leg forward. The next costume block from the Looks category swaps between them. Swap repeatedly while the sprite slides along, and the eye reads it as walking. The same principle drives a flip book whose pages are thumbed quickly.

One snag is almost guaranteed. A child puts next costume inside a forever loop and the cat seems to vibrate wildly. The computer swaps costumes dozens of times a second, far faster than anyone can see. The fix is a wait 0.2 seconds block inside the loop. That small number is a learner's first timing tool, and trying values from 0.1 to 0.5 shows how much it changes the feel of the movement.

Wait blocks also pace conversations between characters. One sprite says hello for 2 seconds while the second sprite waits 2 seconds and then replies. Children who build a three-turn dialogue usually work out for themselves that the seconds on both sprites have to match. In the costume editor they can also redraw a face or duplicate a costume and tweak it, turning sad, happy and surprised into three costumes the story can call on.

An if-then block runs its contents only when a condition is true. Conditions are the pointy-ended blocks, and most come from the Sensing category: touching a particular sprite, touching a particular colour, space key pressed, or mouse down. There is also an if-then-else block with two branches, for instance showing a winner message when the score reaches 10 and carrying on with the game when it has not.

The single most common mistake is placing if-then outside a loop. The program checks the condition once, at the instant the green flag is clicked, when the fruit and basket are nowhere near each other, and then never checks again. Learners need to see that the if-then belongs inside a forever loop so Scratch keeps asking whether the touch has happened yet. Sketching the flow on paper before building often helps.

Colour sensing opens up many simple games. A maze with black walls works with one rule: if touching black, go back to the start. A green finish line can trigger a winning message. Older pupils can combine two conditions with the and or the or block from the Operators category, perhaps adding points only when the basket is touched and the fruit is red.

Once a project has more than two characters, a learner needs a way for one sprite to signal another. Scratch provides the broadcast block and the matching hat block, when I receive. The cat can broadcast a message named dog-turn after it finishes speaking, and the dog starts its reply on receiving that message. This replaces the older trick of matching wait times across two sprites, which falls apart as soon as the dialogue is edited.

Broadcasts also drive level changes and scene changes. When the score reaches 10, the stage broadcasts level-2, the backdrop switches, enemies speed up and the title sprite shows new text. Upper primary pupils building a branching story, where the reader picks the left door or the right door, can use one message per branch. That structure keeps a large project readable.

A common stumbling block is messy message names such as message1, message2 and message3, which leave the child guessing what each one does. Naming messages after their event, like start-market-scene or game-over, makes a real difference when the project is reopened a week later. There is also broadcast and wait, which holds the sending sprite until every receiver has finished its script.