Last month, I made two different Lego creations. I made a tank with a working turret and two mechanical arms.
The tank is remote controlled and the turret can aim up and down, spin 360 degrees, and shoot 11 darts in about two seconds.
One of the mechanical arms has pneumatic (air powered hydraulics) claws, and the other arm is motor-powered linear actuators. Both arms are powered by/connected to a vest. The pneumatic arm is powered by a small compressor using two large, modified manual pumps. I took the springs out of the pumps so one XL motor could run both pumps at once.
Stuff That Jack Built 