"This set was a gift for my son and it was great to go through the build with him the first time. But as someone mentioned earlier, the caps of the boosters do not stay on. My son plays actively with his Lego sets (as he should!), but these caps don't stay on even when I attempt to place them on the tops of the boosters. One of them just pops back off as soon as it is placed on the booster. I think different parts should be used for the caps rather than what is included."