When covering the topic (during my last teaching practice) of 'Estimation' I carried out a very successful lesson.

I set out a number of groups of objects e.g. a cup of paper-clips, a pile of books.

I then asked the children to estimate/guess e.g how many paper-clips in the cup. They wrote down their estimation, the actual number and whether the guess was too many, too little etc.

The children can also estimate measurements (length, volume, capacity), as well as quantity.

This lesson was successful as it was both practical and introduced the language in a simple way.