Melochas Boneh
Is there any problem to build legos on Shabbos? The main issue involved is the melocha of boneh or makeh b’patish. Even though we hold that there is no building by small objects (as opposed to big objects or objects attached to the ground), but to create something from nothing by combining different parts would be forbidden. The question is if legos qualify to be considered building something new. Firstly, is the connection between the different pieces a strong connection that will hold together? This would probably depend on the size of the pieces. The bigger pieces that are meant for younger children that do not hold strong would not be considered boneh.
Even if the pieces do not hold together strong there is another issue to be considered. Is there any permanence to the structure being built? Something in which the normal use is to take apart after building, many poskim say is not considered building. This is usually the case by legos.
An older child who knows how to build complex structures and also plans on keeping them together for a while, would be considered boneh. To build these complex structures might also be a problem of koseiv or ohel, two other meloches.
Rav Shlomo Zalman Aurbach zt”l was of the opinion that whatever is being built with lego is always seen as being just a toy rather than building an actual object. He did however add that since the child considers it to be building something, lego should be avoided for chinuch reasons. A parent does not have to stop their child from playing.

abbi Tzvi Aryeh Hyman, Mir Yeshiva
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