riley elwood asked "can you make a rubber band powered car out of 4 toothpicks 2 straws and 4 bottle caps". The video is virtually real time with no speeded up sections so the total build time was only about 15 minutes.
The back toothpick simply has a very short piece of toothpick glued to it so it forms a hook to hold the band as you turn the axle.
Measurements: How long is your straw? You need holes for the axles to go through, they need to be the same position on both straws or the car will not go straight. You need holes in both straws for the cross-members to go through, they need to be the same position on both straws or the car chassis will not be straight. Other than that you can put the holes anywhere you like on the straws. Probably as near to the end of the straw as you dare without weakening the straw. If you watch the video carefully you can see I am using a ruler that is marked in centimetres and it looks like I am probably marking the holes about one centimetre from the end and then the next hole about one centimetre from that.
Making the holes might be an issue. Melting the plastic is better than trying to drill it or push a sharp spike through but it can be done. Using a hot wire or soldering iron may be a safety problem but you get a stronger hole. In the straws it is important that the holes for the cross members are a tight fit and the holes for the axles are very loose. The holes in the wheels need to be a very tight fit.
"Does adding the clay make the car move further? If so, what is the science behind it?"
The answer was already explained in the video. The car was too light so the wheels lacked grip and slipped on the floor, spinning round without moving forwards. Adding the clay to the wheels made them heavier so they had more grip, stopping them slipping, so they could drive the car forwards.
The science is about friction.
2 drinking straws
4 bottle caps
rubber bands or loom bands
Modelling Clay for extra weight
Hot Glue Gun - If you get the relevant holes nice and tight you don't need hot glue but I like to use it.
Soldering iron - At 03:45 I explain I am using my soldering iron to melt the holes. You could just use a sharp nail or a drill but using the soldering iron means there is no chance of the plastic splitting, you need a steady hand to do it.
Snowman plays247 asked "What other types of materials can you use other than straws?". I have split the reply video over 4 episodes and placed it in it's own playlist. https://www.youtube.com/playlist?list=PLA5a2xPRSrB0F2PNcApzXxp_nFkUjnCMt
Here is a selection of related playlists
Rubber Band Power Cars https://www.youtube.com/playlist?list=PLA5a2xPRSrB3g5RiX8Kf_uUBOL0Tb7AS0
Rubber Band Power playlist https://www.youtube.com/playlist?list=PLA5a2xPRSrB1otXh0w36OwsYIHZznUWp6
Brakes on Rubber Band Cars playlist https://www.youtube.com/playlist?list=PLA5a2xPRSrB3MyidvKMT3DqU1vNy9Atsf
Carrying Dropping Pushing Pulling Stopping playlist https://www.youtube.com/playlist?list=PLA5a2xPRSrB04L6RpUAs5akmK3jqmahPL
Rubber Band Wikipedia https://en.wikipedia.org/wiki/Rubber_band
Size 64 rubber band measure 3 1/2"x 1/4"x 1/32" or 89mm x 6mm
You can help keep my channel running with a dollar for coffee and cakes at https://www.patreon.com/GrandadIsAnOldMan
If you don't want to use Patreon you can buy me a coffee here instead http://ko-fi.com/grandadisanoldman
Merchandise on RedBubble http://www.redbubble.com/people/grandadiaom/portfolio
Merchandise on Cotton Cart http://grandadisanoldman.cottoncart.com/
Oldest videos https://www.youtube.com/c/GrandadIsAnOldMan/videos?flow=grid&view=0&sort=da
Most Popular https://www.youtube.com/c/GrandadIsAnOldMan/videos?sort=p&flow=grid&view=0
Newest videos https://www.youtube.com/c/GrandadIsAnOldMan/videos?sort=dd&view=0&flow=grid
I have thousands of videos on YouTube, to find one try my search guides http://uvideoplay.com/video/_Y2VCU09vT0hjZ3c.html or http://uvideoplay.com/video/_Wmo0VWd3a3dZUms.html
Put a rubber band on it https://www.youtube.com/channel/UChTttryzps7BEkm-xk_cKUA